MCAT General Chemistry Questions

Specific heats (J/g-degrees C): Fe = 0.44, Au = -0.13, Al = 0.90, Cu = 0.39 In an experiment, it was found that 6 kJ of heat were required to raise the temperature of a sample of copper by 15 degrees C. If the experiment was repeated with a gold sample of the same mass, how much heat would be required to achieve the same temperature change? A) 2 kJ B) 4 kJ C) 12 kJ D) 18 kJ

A) 2 kJ Since the specific heat for Au is one-third as large as the specific heat for Cu, one-third as much heat will be required to get the same temperature change. One-third of 6 kJ is 2 kJ.

Cl2 + 2Br- --> Br2 + 2Cl- In the above reaction: A) Cl2 is the oxidizing agent and Br- is oxidized B) Cl2 is the oxidizing agent and Br- is reduced C) Cl2 is the reducing agent and Br- is oxidized D) Cl2 is the reducing agent and Br- is reduced

A) Cl2 is the oxidizing agent and Br- is oxidized Cl2 is the elemental form of Cl, so it has an oxidation state of 0. the oxidation state of Cl in Cl- is 1. Br starts with an oxidation state of -1 as Br-, and ends in its elemental form as Br2 with an oxidation state of 0. Chlorine gains electrons, so it is reduced. Bromine loses electrons, so it is oxidized. Choices B and D can be eliminated. Since Cl2 is reduced, it is the oxidizing agent.

A student has a block of an unknown solid in the lab. Which of the following will most likely melt the block? I. heating the solid at a constant pressure II. compressing the solid at a constant temperature III. accelerating the solid to high speeds to increase its KE A) I only B) I and II only C) I and III only D) I, II, and III

A) I only Heating the solid will raise its temperature and will eventually melt it. Compressing the solid will raise the pressure on the solid which will most likely keep it a solid. A few substances like water will melt under pressure, but for most substances, pressure changes liquid to a solid. It is the random KE of the molecules of a solid, and not the uniform translational motion KE of the solid that increases its temperature and causes it to melt.

Which of the following expressions represents the solubility product for Cu(OH)2? A) Ksp = [Cu2+][OH-]2 B) Ksp = [Cu2+]2[OH-] C) Ksp = [Cu2+]2[OH-]2 D) Ksp = [Cu2+][OH-]

A) Ksp = [Cu2+][OH-]2 The solubility product is created by multiplying the concentrations of the products of the solvation while turning the coefficients into exponents. The reactant is not included because it is a pure solid.

Which of the following most likely represents the correct order of ion size from greatest to smallest? A) O2-, F-, Na+, Mg2+ B) Mg2+, Na+, F-, O2- C) Na+, Mg2+, O2-, F- D) Mg2+, Na+, O2-, F-

A) O2-, F-, Na+, Mg2+ This is an isoelectronic series, which means that the number of electrons on each ion is the same. In an isoelectronic series, the nuclear charge increases with increasing atomic number and draws the electrons inward with greater force. The ion with the fewest protons produces the weakest attractive force on the ions, so it has the largest size. Oxygen has the smallest atomic number, so it has the fewest protons, and therefore will have the greatest ion size.

What is the reducing agent in the following reaction? 2HCl + Zn --> ZnCl2 + H2 A) Zn B) Zn2+ C) H+ D) Cl-

A) Zn Zinc begins with an oxidation state of 0 as elemental Zn and ends with an oxidation state of +2 in ZnCl2. The elemental Zn gives up its electrons as it is oxidized. Because Zn gives up its electrons, it is the reducing agent.

The mass number of an atom undergoing radioactive decay will remain unchanged in each of the processes below EXCEPT: A) alpha decay B) beta decay C) electron capture D) gamma decay

A) alpha decay In alpha decay, a particle identical to a helium nucleus is released, and therefore, the mass number decreases by 4. None of the other processes change the mass number.

Which of the following is amphoteric? A) an amino acid B) H2SO4 C) NaOH D) HF

A) an amino acid An amino acid can act as an acid or a base depending on the pH. Although the conjugate base of sulfuric acid is amphoteric, sulfuric acid cannot accept a proton and is not amphoteric. NaOH is a base, and cannot act as an acid in any way. HF is restricted to behaving as an acid. It would never become H2F.

2SO2 (g) + O2 (g) <--> 2SO3 (g) Standard enthalpy of formation = -198 kJ If the above reaction was at equilibrium inside of a closed container, decreasing the volume of the container would cause which of the following? A) an increase in the formation of sulfur trioxide B) a decrease in the formation of sulfur trioxide C) no effect on the formation of sulfur trioxide D) the effect cannot be determined with the information provided

A) an increase in the formation of sulfur trioxide Decreasing the volume causes an increase in pressure (PV = nRT). An increase in pressure favors a shift towards the side with less moles of gas, which are the products in this reaction. As a result, the formation of SO3 will increase.

Consider two gases A and B with different molecular weights in fixed containers of the same volume and pressure. All of the following are true EXCEPT: A) both gases have the same average KE B) the volumes of a molecule of gas A and a molecule of gas B are equal C) the average force exerted on the container by gas A is the same as that exerted by gas B D) the molecules of gas A exert no forces on each other when not colliding

A) both gases have the same average KE These two containers of equal mass have different molecular weights, so there must be a different number of moles of each gas in each container. If pressure and volume are constant, then by the ideal gas law, the temperatures of the two gases must be different. Average KE for a gas is proportional to the gas's temperature. The other answers are assumed to be true for all ideal gases.

Aluminum only has one oxidation state, while chromium has several. Which of the following is the best explanation for this difference? A) electrons in the d orbitals of Cr may or may not be used to form bonds B) electrons in the p orbitals of Cr may or may not be used to form bonds C) electrons in the d orbitals of Al may or may not be used to form bonds D) electrons in the p orbitals of Al may or may not be used to form bonds

A) electrons in the d orbitals of Cr may or may not be used to form bonds Based on Cr's position in the periodic table, it can be inferred that it is a transition metal. As such, the electrons in its d orbitals have the ability to move into valence orbitals and form bonds. Depending on the number of electrons that move from the d orbitals, its oxidation state can vary considerably. Note that aluminum, like all elements in the third period and higher, has d orbitals. However, unlike chromium, Al does not, in its ground state, have electrons in its d orbitals; its configuration is [Ne]3s23p1. Therefore, C can be eliminated.

Suppose that you constructed a simple barometer using water instead of mercury. Approximately how high would the water raise in the tube at sea level? (Note: assume Patm = 1 atm = 101,325 Pa = 760 mmHg. The specific gravity of mercury = 13.6) A) 1.3 m B) 10 m C) 13 m D) 100 m

B) 10 m There are two possible approaches. First, the height of the substance in the tube can be calculated using Patm = ρgh. 101,325 Pa = (1000 kg/m3)(10 m/s2)h, so h approximately equals 10 m. Also remember that 1 atm = 760 mmHg. Mercury is 13.6 times denser than water, so water will rise 13.6 times as high, or approximately 14 times higher. 760 mmHg is about 10,640 mmH2O = 10.64 m. The closest answer choice is B.

A spoon is plated with silver in an electrolytic process where the half reaction at the cathode is: Ag+ (aq) + e- --> Ag (s) with E° = 0.8 V. If the current "i" is held constant for "t" seconds, which of the following expressions gives the mass of silver deposited on the spoon? A) 107.8itF B) 107.8(it/F) C) 107.8(i/tF) D) 107.8(iF/t)

B) 107.8(it/F) Use units to solve the problem. We want to go from current to grams. Current is Coulombs/sec. F is Coulombs/mole of electrons. From the balanced equation, we can see that for every mole of electrons, there is one mole of silver. The molecular weight of silver is 107.8 g/mol. (Coulombs/sec) x (sec) x (mol/Coulombs) x (grams/mol) = grams i x t x (1/F) x 107.8 = grams

Suppose that a student prepares two acidic solutions. Solution A has H+ concentration of 6.0x10^-5. Solution B has H+ concentration of 1x10^-7. The pH of solution A differs from that of solution B by: A) 1.3 B) 2.8 C) 3.7 D) 5.0

B) 2.8 The pH of solution B is 7. The pH of solution A is between 4 and 5. This is because a hydrogen ion concentration of 1.0x10^-5 mol/L would result in a pH of 5, and a concentration of 1.0x10^-6 mol/L would result in a pH of 6. Additionally, if the hydrogen ion concentration is greater than a known value, then the value for the pH will be smaller, and vice versa. The only pH difference between 7 and 4 -5 is choice B.

A natural sample of carbon contains 99% of C-12. How many moles of C-12 are likely to be found in a 48.5 gram sample of carbon obtained from nature? A) 1 B) 4 C) 12 D) 49.5

B) 4 Solving this problem does not require complicated calculations. Simply assume that 100% of the sample is 12-C. The molecular weight of 12-C is 12 g/mol. 48.5 g x 12 g/mol = ~4 mol.

The acid dissociation constant of HC6H7O6 is 8.0x10^-5. If a solution contains equal concentrations of HC6H7O6 and C6H7O6-, what will be the pH of the solution? A) 3.0 B) 4.1 C) 5.3 D) 9.0

B) 4.1 The Henderson-Hasselbalch equation shows that when a weak acid and its conjugate base are present in a solution in equal amounts, the pH will be equal to the pKa of the conjugate acid. This is because the log(1) = 0. The negative logarithm of 8.0x10^-5 is between 4 and 5, narrowing down the answer to choice B.

The acid dissociation constant for HBrO is 2x10^-9. What is the base dissociation constant for BrO-? A) 5x10^-5 B) 5x10^-6 C) 5x10^-7 D) 5x10^-8

B) 5x10^-6 BrO- is the conjugate base of HBrO, so the base dissociation constant Kb can be found by dividing. (1x10^-14)/(2x10^9) = 0.5x10^-5, which is the same as 5x10^-6.

The following is a list of acid dissociation constants for 4 acids: Acid 1: Ka = 1.2x10^-7 Acid 2: Ka = 8.3x10^-7 Acid 3: Ka = 3.3x10^-6 Acid 4: Ka = 6.1x10^-3 Which acid should be used to manufacture a buffer at pH of 6.1? A) Acid 1 B) Acid 2 C) Acid 3 D) Acid 4

B) Acid 2 A buffer is made from equal amounts of acid and its conjugate. The buffer works best when the pH = pKa. This can be shown on a titration curve, as the graph is most horizontal when pH = pKa. This means adding base or acid will have a small effect on the solution, or in other words, the solution is buffered well. The -log(8.3x10^-7) is between 6 and 7. 8.3 is close to 10, making the pKa closer to 6.

Which of the following species has an unpaired electron in its ground-state electronic configuration? A) Ne B) Ca+ C) Na+ D) O2-

B) Ca+ Atoms and ions with electron configurations identical to those of noble gases do not have any unpaired electrons in their ground state. Ne, Na+, and O2- all have the same electron configuration, [Ne], and therefore have no unpaired electrons in their ground state. Ca+, by contrast, has a ground state configuration of [Ar]4s1, and has one unpaired electron in its 4s subshell.

Which of the following is the strongest base in aqueous solution? A) Cl- B) NH4+ C) F- D) Br-

C) F- NH4+ is an acid. The strongest base is the conjugate of the weakest acid. This is because KaKb = Kw. The respective strengths of the members of any conjugate pair are inversely related. What this question is really asking is to find the option with the weakest conjugate acid, and that is F- (HF is the conjugate acid). As the bond strength of the halogen and the hydrogen increases (highest in HF), the strength of the acid decreases.

Which of the following molecules has the greatest dipole moment? A) H2 B) O2 C) HF D) HBr

C) HF The dipole moment will be greatest for the atoms with the greatest difference in electronegativity. Based upon periodic trends, H and F will have the greatest difference in electronegativity, and therefore the greatest dipole moment.

Nitrous oxide is prepared by the thermal decomposition of ammonium nitrate. NH4NO3 (s) --> N2O (g) + 2H2O (g) The equilibrium constant for this reaction is: A) [NH4NO3]/[N2O][H2O] B) [N2O][H20]^2/[NH4NO3] C) [N2O][H2O]^2 D) [N2O][H2O]

C) [N2O][H2O]^2 The equilibrium constant is products over reactants with the coefficients as exponents. Choice A can be eliminated because it has the reactant over the products. Additionally, reactants and products in pure liquid and solid phases are not included in the equilibrium expression, which eliminates choice B. Choice D fails to include the coefficient of the water vapor.

23/12-Mg decays into 23/11-Na with the emission of: A) a proton B) an electron C) a positron and a neutrino D) a positron

C) a positron and a neutrino Remember that charge and mass are always conserved during nuclear decay on the MCAT. This means that the sum of the reactant masses and the sum of the reactant charges are equal to the sum of the product masses and product charges. 23 = 23, so there is nothing to worry about to balance the masses in this equation. because the charge number is not balanced, a new charge needs to be introduced into the decay equation to balance it. 12 = 11 + x, so x = +1. A particle with a positive charge and no mass is a positron. A positron must be emitted alongside the decay product of this reaction in order for the reaction to be balanced. during positron emission, neutrinos, which are particles lacking charge and mass are also released, so C is the best answer choice.

When an electron moves from a 2p to a 3s orbital, the atom containing that electron: A) becomes an new isotope B) becomes a new element C) absorbs energy D) releases energy

C) absorbs energy 3s orbitals are at higher energy than 2p orbitals. For an electron to move from 2p to 3s, the atom containing that electron must absorb energy. In order to become a new isotope, the atom must gain or lose a neutron. In order to become a new element, the atom must gain or lose a proton. The atom would release energy if the electron moved to a lower energy orbital.

Specific heats (J/g-degrees C): Fe = 0.44, Au = -0.13, Al = 0.90, Cu = 0.39 If samples of equal mass of all the metals listed are subjected to the same heat source, which metal would be expected to show the LEAST change in temperature? A) iron B) gold C) aluminum D) copper

C) aluminum Aluminum has the largest specific heat, which means that it can absorb the most energy while undergoing the smallest temperature change.

Stomach acid has a pH of approximately 2. Sour milk has a pH of 6. Stomach acid is: A) 3 times as acidic as sour milk B) 4 times as acidic as sour milk C) 100 times as acidic as sour milk D) 10,000 times as acidic as sour milk

D) 10,000 times as acidic as sour milk Each unit of pH is a tenfold increase of acidity, because pH is measured on a logarithmic scale.

Which of the following statements about a galvanic cell is false? A) if E° = 0, a reaction may still be spontaneous depending on the chemical concentrations B) a galvanic cell with a positive potential can perform work C) reduction takes place at the cathode D) a salt bridge balances the charge by allowing positive ions to move to the anode

D) a salt bridge balances the charge by allowing positive ions to move to the anode Positive ions move across the salt bridge to the cathode. This can be memorized by considering that the salt bridge is used to balance the charges. Since negative electrons move to the cathode, positive ions must balance the charge by also moving to the cathode.

The process below takes place in acidic solution. NO2- (aq) --> NO3- (aq) In this process, the oxidation state of nitrogen is: A) reduced from +2 to +3 B) oxidized from +2 to +3 C) reduced from +3 to +5 D) oxidized from +3 to +5

D) oxidized from +3 to +5 The two oxygens in NO2- have a total oxidation number of -4, so nitrogen must have an oxidation number of +3 to get a total of -1 on the polyatomic ion. The three oxygens in NO3- have a total oxidation number of -6, so nitrogen must have an oxidation number of +5 to get a total of -1 on the polyatomic ion. Since the oxidation state is increasing from +3 to +5, electrons are being lost and oxidation is taking place.

The addition of an electron withdrawing group to the alpha carbon of a carboxylic acid will: A) increase the acidity of the proton by making the O-H bond more polar B) increase the acidity of the proton by making the O-H bond stronger C) decrease the acidity of the proton by making the O-H bond more polar D) decrease the acidity of the proton by making the O-H bond stronger

A) increase the acidity of the proton by making the O-H bond more polar The electron withdrawing group will further polarize the O-H bond, and polarization increases acidity in aqueous solution. Even if the O-H bond became stronger, which it does not, this change would result in a weaker acid. Choice C is the opposite of choice A. Choice D is false, as a stabilization of the conjugate base would cause the acid to be stronger, more readily turning into a conjugate base.

AgCl (s) --> Ag+ (aq) + Cl- (aq). During the course of this reaction, both entropy and enthalpy are increased. If the reaction is not spontaneous at a given temperature and pressure, what can be done to make the reaction occur spontaneously? A) increase the temperature B) decrease the temperature C) increase the pressure D) decrease the pressure

A) increase the temperature ΔG = ΔH - TΔS. For a spontaneous reaction, ΔG must be negative. ΔH > 0 and ΔS < 0 for the reaction given. In order for ΔG to be negative, an increase in temperature will increase the negative term of the equation, -TΔS. If T is increased enough, ΔG will eventually switch from positive to negative. Changing the pressure will have no effect on a nongaseous reaction that takes place in a solution.

Nitric acid is produced commercially by oxidation in the Oswald process. The first step of this process is shown below: 4NH3 (g) + 5O2 (g) <--> 4NO (g) + 6H2O (g) A container holds 4 moles of gaseous ammonia, 5 moles of gaseous oxygen, 4 moles of gaseous nitric oxide, and 6 moles of water vapor at equilibrium. Which of the following would be true if the container were allowed to expand at constant temperature? A) initially during the expansion the forward reaction rate would be greater than the reverse reaction rate B) the equilibrium would shift to the left C) the partial pressure of oxygen would increase D) the pressure inside of the container would increase

A) initially during the expansion the forward reaction rate would be greater than the reverse reaction rate Expansion of the container increases the volume and decreases the pressure, so choice D can be eliminated. By Le Chatelier's principle, a decrease in pressure shifts the reaction to the side with more moles of gas. In this problem, the ratio of moles of gas in terms of reactants to products is 9 to 10. As a result, the equilibrium would shift to the right by increasing the forward reaction rate, eliminating choices B and C.

If the expected equivalence point for a titration is at a pH of 8.2, which of the following would be the best indicator for the titration? A) phenolphthalein: Ka = 1.0x10^-8 B) bromthymol blue: Ka = 7.9x10^-8 C) methyl orange: Ka = 3.2x10^-4 D) methyl violet: Ka = 1.4x10^-3

A) phenolphthalein: Ka = 1.0x10^-8 An indicator generally changes color within plus or minus 1 pH point of its pKa. Find the pKa that is closest to 1x10^-8.2, or slightly less than 10^-8. The closest option is phenolphthalein.

Which of the following statements about entropy is false? A) the entropy of a system will always increase in a spontaneous reaction B) entropy is a measure of disorder C) the entropy change of a forward reaction is exactly opposite to the entropy change of the reverse reaction D) entropy increases with temperature

A) the entropy of a system will always increase in a spontaneous reaction The entropy of the universe will increase in a spontaneous reaction, but the entropy of a system may or may not increase. Entropy increases with number, volume, and temperature (all allow for increased disorder). Additionally, the entropy change for the reverse reaction is the opposite sign of the entropy change for the forward reaction of the system because entropy is a state function.

Sulfur dioxide reacts with oxygen in a reversible reaction to form sulfur trioxide (2SO2 + O2 --> 2SO3) with standard enthalpy of formation of -200 kJ/mol. If the temperature at which the reaction takes place is increased, which of the following will occur? A) the rates of the forward and reverse reactions will increase B) only the rate of the forward reaction will increase C) only the rate of the reverse reaction will increase D) the rates of neither the forward nor reverse reactions will increase

A) the rates of the forward and reverse reactions will increase Increasing the temperature increases the energy available to both the forward and reverse reactions, enabling both to more easily overcome the activation energy. As a note, the rate of the reverse reaction will increase more than the rate of the forward reaction. This is because the reaction is exothermic, making heat a product. Based on Le Chatelier's principle, an increase in temperature (heat) will cause a leftward shift by increasing the reverse rate of reaction more than the forward rate.

Which of the following is a type of kinetic energy? A) vibrational energy B) chemical energy C) rest mass energy D) intermolecular energy

A) vibrational energy

Two chemical reactions involving water are shown below: Reaction 1: NH4+ + H2O --> NH3 + H3O+ Reaction 2: NaH + H2O --> Na+ + OH- + H2 Which of the following is true? A) water acts as a base in reaction 1 and as an acid in reaction 2 B) water acts as an acid in reaction 1 and as a base in reaction 2 C) water acts as a base in both reactions D) water acts as neither an acid nor a base

A) water acts as a base in reaction 1 and as an acid in reaction 2 In reaction 1, water accepts a proton to become H3O+, so it is acting as a Bronsted-Lowry base. In reaction 2, water gives up a proton to become OH-, so it is acting as a Bronsted-Lowry acid.

An aqueous solution of 0.1 M HBr has a pH of: A) 0 B) 1 C) 2 D) 14

B) 1 HBr is a strong acid, so it dissociates completely. The concentration of H+ ions will be equal to the concentration of HBr. -log(0.1) = 1.

Which of the following are true statements? I. the heat capacity of a substance is the amount of heat that substance can hold per unit of temperature II. the specific heat for a single substance is the same for all phases of that substance III. when heat is added to a fluid, its temperature will change less if it is allowed to expand A) I only B) III only C) I and III only D) I, II, and III only

B) III only Heat capacity is the amount of energy needed to change the temperature of an object by a given amount. Heat is a transfer of energy. Objects cannot contain heat. Additionally, the same amount of the same substance can have the same amount of energy and be at different temperatures. The only choice that does not contain statement I in it is choice B. Statement II is false. Different phases will typically have different specific heats, though rare exceptions do exist. Statement III is true.

In the reaction below, ammonia and boron trifluoride combine when a coordinate covalent bond is formed between nitrogen and boron. In this reaction, ammonia acts as a: NH3 + BF3 --> H3NBF3 A) Lewis acid B) Lewis base C) Bronsted-Lowry acid D) Bronsted-Lowry base

B) Lewis base In a coordinate covalent bond, one atom donates an electron pair to share with another atom. No proton transfer is involved, so C and D can be eliminated. In this case, ammonia has the unbonded pair to donate to boron, so ammonia is the Lewis base and boron is the Lewis acid. BF3 can be ruled out as the Lewis base because it has no electrons to donate. It only has 3 valence electrons and already has 3 bonds to the fluorine atoms.

Ammonia reacts with water to form the ammonium ion and hydroxide ion as shown: NH3 + H2O --> OH- + NH4+ According to the Bronsted-Lowry definition of acids and bases, what is the conjugate acid of ammonia? A) NH3 B) NH4+ C) OH- D) H+

B) NH4+ The conjugate acid is the molecule after it accepts a proton. In this reaction, NH3 becomes NH4+ after accepting a proton. Therefore, NH3 is the conjugate base, and NH4+ is the conjugate acid.

What is the electron configuration of chromium? A) [Ar] 3d6 B) [Ar] 4s13d5 C) [Ar] 4s23d3 D) [Ar] 4s24d4

B) [Ar] 4s13d5 Even without pre-existing knowledge of the electron configuration of Cr, strongest answer could have been identified by eliminating improbable choices. Choice C can be eliminated because it has the wrong number of electrons. Choice D can be eliminated because Cr ground state electrons exist in the 3d rather than 4d subshell. Given Hund's rule, which states that the most stable arrangement of electrons is the one with the most unpaired electrons, choice B might have seemed more likely to be the best answer.

Which of the following is true for a reaction, if ΔG° = 0 at 298 K? A) the reaction is at equilibrium B) at 298 K and 1 M concentrations of products and reactants, the equilibrium constant equals 1 C) ΔG is also 0 D) the reaction is spontaneous at temperatures greater than 298 K

B) at 298 K and 1 M concentrations of products and reactants, the equilibrium constant equals 1 This question requires knowledge of the equation ΔG° = -RTln(K). This equation is a statement about the relationship between ΔG° and K at a specific temperature. If ΔG° = 0, then K = 1. The standard state for an aqueous solution is 1 M concentrations. Do not confuse ΔG with ΔG°. At equilibrium, ΔG = 0 and ΔG° is given as in the equation above. Choices A and C therefore state the same incorrect assumption.

A PET scan uses a derivative of glucose with a radioactive fluoride-18 atom in place of the 2' oxygen. The radioactive fluoride emits a positron which allows physicians to measure metabolic activity, a key marker for identifying cancerous tumors. Which decay process is exhibited by F-18? A) alpha decay B) beta decay C) gamma decay D) glycolytic decay

B) beta decay The question stem states that fluoride-18 emits a positron. Beta decay is characterized by emission of a positron, or beta particle, from the nucleus as a proton is turned into a neutron. A positron is the antimatter equivalent of an electron. Choice B is the best answer.

2SO2 (g) + O2 (g) <--> 2SO3 (g) Standard enthalpy of formation = -198 kJ For the above reaction, which of the following will most increase the production of heat? A) high pressure and high temperature B) high pressure and low temperature C) low pressure and high temperature D) low pressure and low temperature

B) high pressure and low temperature In order to increase the production of heat, the reaction must shift towards the right and generate products. Increasing the temperature will cause a leftward shift, which is opposite the desired effect, eliminating A and C. The effects of pressure3 change are dependent on the moles of gas of reactants vs. the moles of gas of the products. Looking only at gases, 3 moles of reactants generates 2 moles of products. A decrease in pressure shifts the reaction towards the side with more moles of gas, in this case, the reactants. Therefore, a high pressure and low temperature will increase the production of heat, eliminating D.

Which of the following will increase the vapor pressure of a liquid? A) increasing the surface area of the liquid by pouring it into a wider container B) increasing the KE of the molecules of the liquid C) decreasing the temperature of the liquid D) adding a nonvolatile solute

B) increasing the KE of the molecules of the liquid Molecules break free of the surface of a liquid and add to the vapor pressure when they have sufficient KE to break the intermolecular bonds that hold them together as a liquid. Choice C is exactly the opposite of choice B: decreasing the temperature will decrease KE of the molecules. Increasing the surface area will have no effect on the vapor pressure. Finally, adding a non-volatile solute decreases the vapor pressure of the whole solution.

In reference to the photoelectric effect, which of the following will increase the kinetic energy of a photoelectron? A) increasing the work function B) increasing the frequency of the incident light C) increasing the number of photons in the incident light D) increasing the mass of photons in the incident light

B) increasing the frequency of the incident light KE = hf - phi, in which h is Planck's constant, f is the frequency of incident light, and phi is the work function of the metal. In other words, only the frequency of the incident light and the work function of the metal affect the KE of ejected electrons, so C and D can be eliminated. Note that photons are conventionally held to be massless, so this eliminates D as well. The work function is the minimum energy required to eject an electron from a solid state into a surrounding vacuum. Increasing the work function decreases the kinetic energy of the ejected electron. Only increasing frequency increases KE.

A solution of ethanol and methanol can be thought of as ideal. At room temperature, the vapor pressure of ethanol is 45 mmHg, and the vapor pressure of methanol is 95 mmHg. Which of the following will be true regarding the vapor pressure of a solution containing only ethanol and methanol? A) it will be less than 45 mmHg B) it will be greater than 45 mmHg and less than 95 mmHg C) it will be greater than 95 mmHg and less than 140 mmHg D) it will be greater than 140 mmHg

B) it will be greater than 45 mmHg and less than 95 mmHg In an ideal solution, the vapor pressure will be somewhere in between the vapor pressures of the solute and solvent, depending on their relative mole fractions.

When heat energy is added evenly throughout a block of ice at 0 degrees C and 1 atm, all of the following are true EXCEPT: A) the temperature remains constant until all the ice is melted B) the added energy increases the KE of the molecules C) entropy increases D) hydrogen bonds are broken

B) the added energy increases the KE of the molecules Watch out for the "EXCEPT" in this question. The added energy goes into breaking bonds, not increasing the KE of the molecules, so choice B is not true. As demonstrated by the heat curve, the temperature remains constant until all the ice is melted, so choice A is true. Entropy increases moving to the right on the heat curve and as organized solid transitions to a more fluid flowing liquid. Intramolecular hydrogen bonds are broken as the ice transitions to water.

If the density of a gas is given as ρ, which of the following expressions represents the molecular weight of the gas? A) P/ρRT B) ρRT/P C) nRT/Pρ D) Pρ/RT

B) ρRT/P This is essentially an algebra equation that tests understanding of density and the ideal gas law. Remember that density is mass divided by volume, and the ideal gas law is PV=nRT. Mass is molecular weight times moles (n). Substituting in for mass in the density equation gives us ρ = m/V = n(MW)/V. After some algebra, we have MW = (pV)/n. From the ideal gas law, we know that V/n = RT/P. Substituting RT/P for V/n gives MW = ρRT/P, which is choice B.

What is the oxidation state of sulfur in H2SO4? A) -2 B) +3 C) +6 D) +7

C) +6 Each oxygen has an oxidation state of -2, and hydrogen has an oxidation state of +1. In order for the ion to have a 1- charge, the sulfur must have a +6 oxidation state.

A solution of soapy water has a pH of 10. What is the hydroxide ion concentration? A) 10^-10 M B) 10^-7 M C) 10^-4 M D) 10^-1 M

C) 10^-4 M If the pH is 10, the pOH must be 4. If the pOH is 4, then the hydroxide ion concentration must be 10^-4 M. Choice A gives the correct hydronium ion concentration, but that is not what the question asks for.

The half-life of substance X is 45 years, and it decomposes to substance Y. A sample from a meteorite was taken which contained 1.5% of X and 13.5% of Y by mass. If substance Y is not normally found on a meteorite, what is the approximate age of the meteorite? A) 45 years B) 100 years C) 140 years D) 270 years

C) 140 years Since Y is not normally found on the meteorite, we assume that all of the Y came from the decomposition of X. The percentage of the sample that was X at the birth of the meteorite must have been the sum of the percentages of Y and X, or 15%. The percentage of the sample that is X is now only 10% of that (or 1.5%). After the first half-life, there would have been 50% left; after the second half-life 25% left; after 3 half-lives 12.5% left. That is, a little more than 3 half-lives is required to reduce a substance to 10% of its original amount. Each half-life is 45 years. 45 years x 3 half-lives ives 135 years. A little more than three half-lives are used, so the best answer is a little more than 135 years.

A buffered solution has a pH that cannot readily be changed. A buffered solution will be produced by mixing equal volumes of: A) 1M HCl and 1M NaC2H3O2 B) 1M HCl and 1M NaOH C) 1M HC2H3O2 and 1M NaC2H3O2 D) 1M HC2H3O2 and 1M NaOH

C) 1M HC2H3O2 and 1M NaC2H3O2 A buffered solution is formed when equal amounts of a weak acid and its conjugate base are present in solution. Acetic acid is a weak acid and the acetate ion is its conjugate. The rest of the options contain strong acids or bases, and non-conjugate pairs.

I-131 is a radionuclide used in the treatment of thyroid carcinomas. If a sample's activity dropped from 100 mCi to 12.5 mCi in 12 days, what is the half-life of I-131? A) 1.5 days B) 3.0 days C) 4.0 days D) 6.0 days

C) 4.0 days One reliable strategy to solve half-life problems is to write out the remaining activity above each half-life. Knowing that 12 days represents 3 half-lives allows the problem to be solved with division. 12 days/3 hl = 4 days/1 hl.

Which of the following is the equivalence point when the weak acid acetic acid is titrated with NaOH? A) 4.3 B) 7 C) 8.7 D) 14

C) 8.7 The equivalence point of a titration of a weak acid with a strong base will always be greater than 7. This is the same as adding the conjugate base of an acid to pure water. 14 is way too basic. Pure 1M NaOH has a pH of 14. pH 7 is the equivalence point for strong-strong titration.

A 13 g gaseous sample of an unknown hydrocarbon occupies a volume of 11.2 L at STP. What is the hydrocarbon? A) CH B) C2H4 C) C2H2 D) C3H3

C) C2H2 Remember that at STP (0 degrees C and 1 atm), 1 mole of gas occupies 22.4 L. This is found by substituting in 0 degrees C, 1 atm, and 1 mole into the ideal gas law. Since this unknown hydrocarbon occupies half of that volume, 11.2 L, you must have 0.5 moles of the sample. The molecular weight of the hydrocarbon is 13 g/0.5 mol = 26 g/mol. Carbon has a molecular weight of 12 g/mol and hydrogen has a molecular weight of 1 g/mol. The only answer choice with the correct molecular weight is answer choice C, C2H2 (26 g/mol).

Which of the following statements is true concerning the reaction below? 2Al2O3 + 3C --> 4Al + 3CO2 A) both aluminum and carbon are reduced B) both aluminum and carbon are oxidized C) aluminum is reduced and carbon is oxidized D) carbon is reduced and aluminum is oxidized

C) aluminum is reduced and carbon is oxidized Aluminum in Al2O3 has an oxidation state of +3 and has an oxidation state of 0 as elemental Al. Carbon begins with an oxidation state of 0 as elemental C and ends with an oxidation state of +4 in CO2. Remember, LEO roars GER. Al gains electrons (+3 to 0) so it is reduced, and carbon loses electrons (0 to +4) and is oxidized. Choices A and B can be eliminated because in a redox reaction, one atom is always oxidized and another atom is always reduced.

Which of the following increases with increasing atomic number within a family on the periodic table? A) electronegativity B) electron affinity C) atomic radius D) ionization energy

C) atomic radius Only atomic radius increases going down a column. Size of an atom is determined not only by effective nuclear charge, but also by number of electron shells. Moving down a column, the number of electron shells increases. Because the additional shells require additional space, atomic radius increases as number of electron shells increases.

By definition, a Lewis base: A) donates a proton B) accepts a proton C) donates a pair of electrons D) accepts a pair of electrons

C) donates a pair of electrons By definition, a Lewis base donates a pair of electrons. A Lewis acid accepts electron pairs. Meanwhile, Bronsted-Lowry acids donate protons and Bronsted-Lowry bases accept protons. Generally, bases will become more positive after the reaction (donate electrons, accept protons), and acids will become more negative (accept electrons, donate protons).

All of the following are forms of internal energy EXCEPT: A) rotational energy B) vibrational energy C) gravitational energy D) translational energy

C) gravitational energy Internal energy is the energy of molecules on a microscopic scale -- all forms of energy excluding those related to the motion of the system as a whole. Gravitational energy is an example of potential energy at the macroscopic level, not the microscopic level.

A reaction quotient of Q > K indicates that the reaction: A) will undergo a rightward shift to increase the concentration of the products B) has a forward reaction rate greater than the reverse reaction rate C) has lower concentrations of reactants than the expected equilibrium concentrations D) favors the formation of products over the formation of reactants

C) has lower concentrations of reactants than the expected equilibrium concentrations Q is the reaction quotient given by [products]/[reactants]. If Q is greater than K, the reaction has a greater concentration of products than would be expected at equilibrium, and a smaller concentration of reactants, making C the right answer. As a result, the reaction would undergo a leftward shift to increase the concentration of reactants, eliminating choice A. In order to do so, the reverse reaction rate would need to be greater than the forward reaction rate, eliminating choice B. Whether the reaction favors the formation of products or reactants cannot be determined without knowing the value of K, eliminating choice D.

Immediately upon bringing a hot piece of metal into a room, the heat is felt from 5 meters away. This heat transfer is probably occurring through: A) convection B) transduction C) radiation D) conduction

C) radiation Heat, which is the movement of thermal energy, occurs via conduction, convection, or radiation. Conduction is transfer via molecular collisions, or physical contact, and conduction through the air is very inefficient and would take a very long time. Convection is transfer of thermal energy via fluid movements, such as air current or a breeze, which is not mentioned in the question. Radiation is transfer via electromagnetic waves or light. The speed of light is 3x10^8 m/s, which would allow for the heat to be felt instantaneously.

Using a bomb calorimeter, the change in energy for the combustion of one mole of octane is calculated to be -5.5x10^3 kJ. Which of the following is true concerning this process? A) since no work is done, the change in energy is equal to the heat B) since there is no work, the change in energy is equal to the enthalpy C) since work is done, the change in energy is equal to the heat D) the work done can be added to the change in energy to find enthalpy

C) since work is done, the change in energy is equal to the heat

Which of the following best explains why sulfur can make more bonds than oxygen? A) sulfur is more electronegative than oxygen B) oxygen is more electronegative than sulfur C) sulfur has 3d orbitals not available to oxygen D) sulfur has fewer valence electrons

C) sulfur has 3d orbitals not available to oxygen Only elements in the third and higher periods of the periodic table can form more than 4 bonds. Second period elements, including oxygen, have four valence orbitals (one 2s and three 2p) with which to form bonds. By contrast, third period elements like sulfur can form bonds with not only one 3s and three 3p, but also five 3d orbitals. Remember that the second quantum number can have any integer value from 0 to n-1, so when n = 3, three subshells (s, p, and d) are available.

All of the following are true concerning a reaction at equilibrium EXCEPT: A) the rate of the forward reaction equals the rate of the reverse reaction B) there is no change in the concentrations of either products or reactants C) the activation energy has reached zero D) the Gibbs free energy has reached a minimum

C) the activation energy has reached zero Equilibrium is defined as the condition where the forward reaction rate equals the reverse reaction rate, eliminating choice A. Although reactants and products are being formed and consumed at equilibrium, there is no change in their concentrations, eliminating choice B. Gibbs free energy is given by ΔG = ΔG* + RTln(Q), where Q = K at equilibrium. Therefore ΔG = 0 and is a minimum at equilibrium, eliminating choice D. The activation energy is a property of the reaction itself and does not change during the reaction, making C the best answer.

A polar solute is poured into a container with a nonpolar solvent. Which of the following best describes the reaction? A) the strong dipoles of the polar molecules separate the weak bonds between the nonpolar molecules B) the dipoles of the polar molecules are too weak to break the bonds between the nonpolar molecules C) the instantaneous dipoles of the nonpolar molecules are too weak to separate the bonds between the polar molecules D) the instantaneous dipoles of the nonpolar molecules separate the bonds between the polar molecules

C) the instantaneous dipoles of the nonpolar molecules are too weak to separate the bonds between the polar molecules If a polar solute and nonpolar solvent are mixed, no solution will be formed (like dissolves like). Choices A and D describe what would happen if this could successfully form a solution. Polar molecules have stronger dipoles, so choice B is not true.

The values of all of the following are reversed when a reaction is reversed EXCEPT: A) enthalpy change B) Gibbs free energy change C) the rate constant D) reaction potential

C) the rate constant Rate constants are dependent on not only temperature, but also frequency factor, which accounts for the frequency of favorably oriented collisions, and activation energy. As a result, their magnitudes differ for forward and reverse reactions, and choice C is the best answer. By contrast, the values of change in enthalpy, change in Gibbs free energy, and reaction potential are all equal in magnitude but opposite in sign for forward and reverse reactions.

Which of the following is true concerning a reaction that begins with only reactants and moves toward equilibrium? A) the rates of the forward and reverse reactions decrease until equilibrium is reached B) the rates of the forward and reverse reactions increase until equilibrium is reached C) the rate of the forward reaction decreases and the rate of the reverse reaction increases until equilibrium is reached D) the rate of the forward reaction increases and the rate of the reverse reaction decreases until equilibrium is reached

C) the rate of the forward reaction decreases and the rate of the reverse reaction increases until equilibrium is reached Initially, the reaction contains only reactants and no products, so the reverse reaction rate begins at 0. As the reactants are consumed, the forward reaction rate decreases while the reverse reaction rate increases. At equilibrium, the rate of the forward reaction will be equal to the rate of the reverse reaction, so the forward decreases and the reverse increases until equilibrium, making C the right answer.

A galvanic cell is prepared with solutions of Mg2+ and Al3+ ions separated by a salt bridge. A potentiometer reads the difference across the electrodes to be 1.05 V. The following standard reduction potentials at 25 degrees C apply: Al3+ + 3e- --> Al E° = -1.66 V Mg2+ + 2e- --> Mg E° = -2.37 V Which of the following statements is true concerning the galvanic cell at 25 degrees C? A) magnesium is reduced at the cathode B) the concentrations of ions are 1 M C) the reaction is spontaneous D) for every aluminum atom reduced, an equal number of magnesium atoms are oxidized

C) the reaction is spontaneous Reactions in galvanic cells are always spontaneous, so their electric potentials are always positive. To find the reaction for this cell, we must flip the more negative half reaction, in this case Mg, to get a positive electric potential (+2.37 V). Now we have a spontaneous cell with a positive electric potential (-1.66 V + 2.37 V = 0.71 V). We also have to multiply the aluminum reactin by 2 and the magnesium reaction by 3 to balance the electrons that are being produced. Notice however that we do not multiply their potentials. The final equation would be 2Al3+ + 3Mg --> 3Mg2+ + 2Al. Since 2 Al atoms are reduced for every 3 Mg atoms oxidized, choice D can be eliminated. We are given that the electrical potential for this cell is 1.05 V. Since it does not equal the electric potential that we calculated, the conditions must not be standard, so choice B can be ruled out. Magnesium is oxidized, so choice A can also be eliminated.

When two pure liquids, A and B, are mixed, the temperature of the solution increases. All of the following must be true EXCEPT: A) the intermolecular bond strength in at least one of the liquids is less than the intermolecular bond strength between A and B in solution B) the reaction is exothermic C) the vapor pressure of the solution is less than both the vapor pressure of pure A and pure B D) the rms velocity of the molecules increases when the solution is formed

C) the vapor pressure of the solution is less than both the vapor pressure of pure A and pure B The vapor pressure of solution will be in between the vapor pressures of the two pure substances, so choice C is not true. We are told that temperature of the solution increases, so we can assume that forming the solution was exothermic. Therefore, the new intermolecular bonds between solvent and solute are more stable than the old intermolecular bonds within the solvent and solute. So A and B are accurate statements. The rms velocity is related to temperature. Since temperature increases, so does rms velocity, and D is an accurate statement.

Solubility products: CaSO4 = 4.93 x 10^-5 Ag2SO4 = 1.2 x 10^-5 PbSO4 = 2.13 x 10^-8 BaSO4 = 1.08 x 10^-10 Na2SO4 dissociates completely in water. From the information given, if Na2SO4 were added to a solution containing equal concentrations of aqueous Ca2+, Ag+, Pb2+, and Ba2+ ions, which of the following solids would precipitate first? A) CaSO4 B) Ag2SO4 C) PbSO4 D) BaSO4

D) BaSO4 The solubility product Ksp is the equilibrium constant for the solvation reaction. As with all equilibrium constants, it is calculated as [products]/[reactants]. Therefore, the smaller the equilibrium constant, the smaller the proportion of products formed compared to the reactants. For solubilities, the smaller the Ksp, the less soluble the compound. The least soluble compound will be the first to precipitate. Here BaSO4 has the smallest Ksp, so it will be the first to precipitate.

Cancers of the bone can be treated with the isotope radium-233. After this isotope undergoes two rounds of alpha decay and one round of beta decay, the remaining substance will possess chemical properties most similar to: A) Na B) Mg C) O D) Cl

D) Cl The chemical properties of an atom are primarily determined by the configuration of its valence electrons, which is most similar among elements of the same column, or group, of the periodic table. The best answer will be the element that is in the same group as the radioactive decay product. When an isotope undergoes alpha decay, it releases a helium nucleus comprised of 2 protons and 2 neutrons, and two rounds of this decay cause the element to lose 4 protons, decaying from 233/88-Ra to 229/86-Rn to 225/84-Po. In beta decay, the loss of a beta particle, 0/-1-B causes the atomic number to increase by 1: 84 - (-1) = 85. The chemical identity of the decay product after three decay processes will be at 225/85-At. This falls in the halogen group, and of the answer choices, only Cl- is in this family.

All of the following statements regarding HCO3- are true EXCEPT: A) HCO3- can act as a Bronsted-Lowry acid B) HCO3- can act as a Lewis base C) HCO3- is amphoteric D) HCO3- is a polyprotic acid

D) HCO3- is a polyprotic acid HCO3- can act as a Bronsted-Lowry acid and give up a hydrogen ion to become CO32-. It can also act as a Lewis base and donate an electron pair to a hydrogen ion to become H2CO3. It is amphoteric because it can act as an acid or base. It is not polyprotic because it only has one hydrogen. Polyprotic acids will, as the name suggests, always have more than one hydrogen ion that they can give away.

Which of the following gas properties is needed to calculate the work done by an expanding gas? I. the initial and final pressures II. the initial and final volumes III. the path followed during the expansion A) I only B) II only C) I and II only D) I, II, and III

D) I, II, and III The work done by a gas is given by w = PΔV. The final and initial volumes would be needed to calculate the change in volume. The equation stated is simplified, because a change in pressure would require calculus. Nonetheless, a change in pressure would also contribute to the work done by the gas. Work is a path function, not a state function, meaning it is dependent on the pathway. As a result, I, II, and III must be known to calculate the PV work (work done by a gas).

Which of the following salts is the most basic? A) NaClO3 B) NH4Cl C) KBr D) NaCN

D) NaCN CN- is the conjugate base of a weak acid. Choices A, B, and C are the conjugates of strong acids, and therefore weaker bases. This is due to the inverse relationship between the strength of conjugates.

Po-216 undergoes two alpha decays and two beta decays to form: A) Tl-208 B) Ra-224 C) Pb-212 D) Pb-208

D) Pb-208 Each of the answers shows an isotope of an element and its associated mass number. Recall that mass number is the sum of the numbers of protons and neutrons. Because atomic number is a defining characteristic of an element, it can be inferred that each element has an atomic number equal to that shown on the periodic table. Therefore, 216-Po has a mass number of 216 and an atomic number of 84. Recall that alpha decay is the loss of an alpha particle consisting of two protons and two neutrons, and beta decay is the conversion of a neutron into a proton, electron, and neutrino. In order to determine the identity of the resulting atom, consider changes to the atomic number. Each alpha decay results in the loss of 2 protons, meaning the new element has atomic number 82; the resulting atom is Pb. Then consider changes that occur to the mass number. Each alpha decay results in a loss of 4 mass units whereas beta decay does not change the mass of the atom. That means that there is a net loss of 8 mass units, so the isotope has a mass number of 208.

Which of the following is the Ka for the conjugate base of carbonic acid? A) [H2CO3]/[OH-][HCO3-] B) [OH-][HCO3-]/[H2CO3] C) [H+][H2CO3]/[HCO3-] D) [OH-][H2CO3]/[HCO3-]

D) [OH-][H2CO3]/[HCO3-] Kb is the reaction of the conjugate base with water. The conjugate base of carbonic acid is carbonic acid deprotonated once (HCO3-). OH- and the conjugate acid (H2CO3) will always be in the numerator. Meanwhile, the conjugate base will be in the denominator.

2SO2 (g) + O2 (g) <--> 2SO3 (g) Standard enthalpy of formation = -198 kJ During the formation of sulfur trioxide, an increase in temperature would result in: A) a decrease in the equilibrium concentration of oxygen B) an increase in equilibrium concentration of products C) no change in equilibrium concentration D) a decrease in equilibrium concentration of sulfur trioxide

D) a decrease in equilibrium concentration of sulfur trioxide An exothermic reaction releases heat, which is considered a product. Increasing the temperature disturbs equilibrium by increasing the amount of heat on the product side, resulting in a shift towards the reactants. As a result, the concentration of SO2 and O2 will increase while the concentration of SO3 will decrease.

The condensation of water is shown. H2O (g) --> H2O (l). Which of the following will be positive for the condensation of water at 25 degrees C and 1 atm? A) ΔH B) ΔS C) ΔG D) none of the above

D) none of the above Bonds are formed when water condenses, so energy is released and ΔH is negative. The water molecules become less random, so ΔS is negative. Condensation occurs spontaneously at 25 degrees C (water is a liquid at room temperature), so ΔG is negative. Notice that this question can be answered without any numbers.

As the temperature of hydrogen gas in a sealed container is increased, which of the following occurs? A) a decrease in the random translational energy, decreasing pressure B) a decrease in the random translational energy, increasing pressure C) an increase in the random translational energy, decreasing pressure D) an increase in the random translational energy, increasing pressure

D) an increase in the random translational energy, increasing pressure The kinetic energy of molecules of a gas is directly proportional to temperature, as given by KE = 3/2RT. Therefore, increasing temperature will increase the random translational energy of the gas molecules, eliminating A and B. If the molecules have a higher kinetic energy, they would be moving faster, hitting the walls of the container more frequently, causing an increase in pressure. Another way to determine the effect of temperature on pressure is to use the ideal gas law (PV = nRT). Increasing the temperature while holding the volume constant would cause the pressure to increase, eliminating choice C.

Which of the following describes a reaction that is always spontaneous? A) increasing enthalpy and increasing entropy B) decreasing enthalpy and decreasing entropy C) increasing enthalpy and decreasing entropy D) decreasing enthalpy and increasing entropy

D) decreasing enthalpy and increasing entropy According to the reaction given by Gibbs free energy, ΔG = ΔH-TΔS, a reaction will be spontaneous (ΔG < 0) if the enthalpy of the system decreases (ΔH < 0) and the entropy of the system increases (ΔS > 0)

At STP, one liter of which of the following gases contains the most molecules? A) H2 B) He C) N2 D) each gas contains the same number of molecules at STP

D) each gas contains the same number of molecules at STP Unless otherwise indicated, assume that all gases behave ideally. The ideal gas law states that n = PV/RT. Therefore, at standard temperature and pressure, equal volumes of ideal gases contain the same number of gas molecules, ruling out choices A, B, and C. If these volumes contain the same number of moles, then they will contain the same number of molecules as well; the number of molecules equals Avogadro's number times the number of moles.

The air we breathe is approximately 21% O2 and 79% N2. If the partial pressure of nitrogen in air is 600 torr, then all of the following are true EXCEPT: A) the mole fraction of nitrogen in air is 0.79 B) the mass of nitrogen in a 22.4 L sample of air is 22.1 grams at 0 degrees C C) the partial pressure of oxygen is approximately 160 torr D) for every 21 grams of oxygen in an air sample, there are 79 grams of nitrogen

D) for every 21 grams of oxygen in an air sample, there are 79 grams of nitrogen 1 atm is equivalent to 760 torr. Since the partial pressure of nitrogen is 600 torr, the mole fraction of nitrogen is 0.79. PN2 = Ptot(XN2) --> XN2 = PN2/Ptot 600 torr/760 torr = 0.79 Since this is the same percentage as the percentage given in the question stem, we know that those percentages are by particle and not by mass. Choice D would be true if the percentages were based on mass. Choice A is true based on our calculations. Choice B is true as it restates what we know about STP. Since the two partial pressures have to add to the total, the partial pressure of oxygen is 760 torr - 600 torr = 160 torr.

All of the following are always true concerning redox reactions except: A) an atom in the reducing agent is always oxidized B) if reduction takes place, so must oxidation C) an atom in the oxidizing agent gains electrons D) if an atom of the reductant loses two electrons, an atom of the oxidant gains two electrons

D) if an atom of the reductant loses two electrons, an atom of the oxidant gains two electrons Consider the reaction 2HCl + Zn --> ZnCl2 + H2. Here, each atom of the reducing agent, zinc, loses two electrons, and the hydrogen atom of the oxidizing agent, HCl, gains one electron. To balance the equation, there must be two hydrogens for each zinc. Choices A and B are both true statements, and state the same thing. Choice C is a true statement based on LEO roars GER.

When solid sodium acetate (NaC2H3O2) is added to pure water, the pH of the solution will: A) decrease, because Na+ acts as an acid B) increase, because Na+ acts as a base C) decrease, because C2H3O2- acts as an acid D) increase, because C2H3O2- acts as a base

D) increase, because C2H3O2- acts as a base Acetate ion (C2H3O2-) is the conjugate base of a weak acid, so it will act as a base in solution. Sodium ion (Na+) is the conjugate acid of a strong base, so it is neutral in solution. The result of a base and a neutral compound in solution is a basic solution, or a pH over 7. This means that the pH will increase from the initial value of 7 due to the contribution from the acetate ion.

A chemical cold pack contains a membrane separating water and ammonium nitrate. When the membrane is broken, the compounds react, cooling the mixture. At standard state, the equilibrium constant K: A) equals 0 B) equals 1 C) is less than 1 D) is greater than 1

D) is greater than 1 When the membrane is broken, the compounds mix and react with outside energy, making the reaction spontaneous (ΔG < 0). Even though this reaction is endothermic (ΔH > 0), recall that a reaction can still be spontaneous based on entropy (ΔG = ΔH - TΔS). At standard state, a spontaneous reaction must have an equilibrium constant K > 1, which is choice D. K < 1 indicates a reaction that is nonspontaneous, eliminating A and C. K = 1 indicates a reaction that is at equilibrium at standard state, which is not true of this reaction, otherwise heat would not be absorbed, so choice B can also be eliminated.

Which of the following is true concerning an air conditioner that sits inside a thermally sealed room and draws energy from an outside power source? A) it will require more energy to cool the room than if part of the air conditioner were outside the room B) it will require more time to cool the room than if part of the air conditioner were outside the rom C) it will require less energy to cool the room than if part of the air conditioner were outside the room D) it cannot cool the room on a permanent basis

D) it cannot cool the room on a permanent basis The second law of thermodynamics states that a heat engine cannot have 100% efficiency in converting heat to work in a cyclical process. An air conditioner is a heat engine running backwards, so an air conditioner must expel more heat than it takes in when it runs perpetually. As time passes, more heat will be generated, making the room warmer, not cooler, so choice B can be eliminated. A specially made air conditioner could initially cool the room, but to cool the room permanently, it must expel the heat to a heat reservoir outside the room. However, the amount of energy required would be the same, which eliminates choices A and C.

Substance A has a greater heat capacity than substance B. Which of the following is most likely true concerning substances A and B? A) substance A has larger molecules than substance B B) substance B has a lower boiling point than substance A C) at the same temperature, the molecules of substance B move faster than those of substance A D) substance A has more methods of absorbing energy than substance B

D) substance A has more methods of absorbing energy than substance B The heat capacity doesn't provide anything about the molecular size of a substance. Heat capacity is the amount of energy required to raise the temperature of a substance by a certain amount. Boiling point is dependent on vapor pressure, not heat capacity. Imagine that substance A is water and substance B is ice. That would satisfy the question stem, but water and ice have the same boiling point. Choice C mistakenly relies upon speed and not KE for temperature. Choice D is the best answer by process of elimination. The more ways that a substance has to absorb energy, the more heat it can absorb with the least change in temperature.

For a particular reversible reaction, the forward process is exothermic and the reverse process is endothermic. Which of the following statements must be true about this reaction? A) the forward reaction will be spontaneous under standard conditions B) the reverse reaction will be spontaneous under standard conditions C) the activation energy will be greater for the forward reaction than for the reverse reaction D) the activation energy will be greater for the reverse reaction than for the forward reaction

D) the activation energy will be greater for the reverse reaction than for the forward reaction Activation energy is greater for an endothermic reaction than for an exothermic reaction.

At atmospheric pressure, the temperature of a pot of boiling water remains at 100 degrees C, when heat is added. The best explanation as to why the added energy does not raise the temperature is that: A) at the boiling point, the large heat capacity of water allows water to absorb the added energy B) the hydrogen bonds of water are strong enough to absorb the added energy without breaking C) as the water becomes steam, the added energy becomes KE of the gas molecules D) the added energy is used to break bonds between water molecules

D) the added energy is used to break bonds between water molecules At the boiling point, any added energy is used to break intermolecular bonds, and not to increase KE, so while the water is boiling, there is no temperature increase.

As temperature is increased, the equilibrium of a gaseous reaction will always: A) shift to the right B) shift to the left C) remain constant D) the answer cannot be determined from the information given

D) the answer cannot be determined from the information given Beware of the "always" in the answer stem. Equilibrium will probably shift with temperature. The direction is dictated by thermodynamics (i.e. if this is an endothermic or exothermic reaction, and the reaction conditions). We need more information.

A negative cell potential indicates which of the following? A) both half reactions are nonspontaneous B) the reduction half reaction potential is greater than the oxidation half reaction potential C) the oxidation half reaction potential is greater than the reduction half reaction potential D) the cell is electrolytic

D) the cell is electrolytic An electrolytic cell has a negative potential, while a galvanic cell has a positive potential.

A rigid container of constant volume is used to store compressed gas. When gas is pumped into the container, the pressure of the gas inside the container is increased and the temperature of the container also increases. Which statement is true of the work done on the container? A) the work is equal to the increase in pressure inside the container B) the work is equal to the increase in temperature inside the container C) the work is equal to the sum of the pressure and temperature increases D) there is no work done on the container

D) there is no work done on the container The work done by or on a gas is given by w = PΔV (constant pressure). Because pressure is not constant, calculus would be needed to solve this problem. However, the container is rigid and has a constant volume, making ΔV = 0, resulting in 0 work.

A concentration cell contains 0.5 M aqueous Ag+ on one side and 0.1 M aqueous Ag+ on the other. All of the following are true EXCEPT: A) electrons will move from the less concentrated side to the more concentrated side B) electrons will move from the anode to the cathode C) as the cell potential moves toward zero, the concentrations of both sides will tend to even out D) ΔG > 0

D) ΔG > 0 A concentration cell is a special type of galvanic cell where the two half reactions in the half cells are the exact reverse of each other. Like a galvanic cell, a concentration cell is always spontaneous, so ΔG < 0. The concentrations in the cell even out at equilibrium, a cell potential of 0. Choices A and B state the same thing, so neither could be the answer.