Chem Qbank
Which of the following is NOT an acid and its conjugate base, respectively? A. Cl+, ClOH B. HSO4-, SO42- C. NH4+, NH3 D. CH3COOH, CH3COO-
Correct answer: A By definition, an acid is a species that donates a H+ ion, and results in the formation of a conjugate base. The only pair that does not meet this criteria is (A). ClOH is an acid, and not a conjugate base. Also, chloride ions do not have a positive charge, and in this example, does not have a H+ ion to donate. Because the Brønsted-Lowry definition of an acid-base reaction is one in which a hydrogen ion (proton) is transferred from an acid to a base, the two always occur in pairs called conjugates. A conjugate acid is the acid formed when a base gains a proton, and a conjugate base is the base formed when an acid loses a proton. For example, HCO3− (aq) + H2O (l) ⇌ CO32− (aq) + H3O+ (aq) CO32− is the conjugate base of HCO3− , a weak acid, and H3O+ is the conjugate acid of H2O, a weak base. Using these principles, it's clear that (B), (C), and (D) all contain conjugate acids with their respective conjugate bases. Of the choices listed, a chloride ion does not have a positive charge, making (A) correct.
Which of the following best pairs the type of titration with its appropriate measurement technique? A. Potentiometric titration uses a voltmeter, while acid/base titration uses a pH meter. B. Potentiometric titration uses a pH meter, while acid/base titration uses a voltmeter. C. Potentiometric titration uses a voltmeter, while acid/base titration uses chromatography. D. Potentiometric titration uses an ohmmeter, while acid/base titration uses a pH meter.
Correct answer: A Potentiometric titrations measure the change in voltage during a titration, since changes in concentration can influence voltage (according to the Nernst equation). Acid/base titrations use a pH meter to assess the pH after the titrant is added. (A) is the correct answer.
An exothermic reaction with a negative change in entropy is run at very low temperatures. Which of the following most likely describes how the reaction will proceed? A. Spontaneously, with a negative ΔG B. Nonspontaneously, with a negative ΔG C. Nonspontaneously, with a positive ΔG D. Spontaneously, with a positive ΔG
Correct answer: A Recall that enthalpy and entropy can be combined in the following equation to yield the Gibbs free energy equation: Δ G = Δ H - T Δ S Also recall that a negative ΔG indicates a spontaneous reaction. From this equation, we learn that exothermic processes (-ΔH) and positive changes in entropy (+ΔS) will yield spontaneous reactions; however, in the case where we have both an exothermic process and a negative change in entropy, we see that temperature will ultimately dictate the spontaneity of the equation in question. From the question stem, we are told that the reaction is run as very low temperatures, indicating that the contribution of the entropy term is relatively very small. Thus, the reaction described in the question stem is primarily governed by the exothermic change in enthalpy, which is a spontaneous process. This is indicated by a negative ΔH, and consequently a negative ΔG, as in (A).
Experimental data for the reaction X + Y ⟶ W + Z is shown in the table below. What is the rate law for this reaction? 1 0.5 0.5 1.25 × 10-2 2 0.5 1.0 2.5 × 10-2 3 1.0 1.0 5.0 × 10-2 A. Rate = 0.05 M-1s-1[X][Y] B. Rate = 0.005 M-1s-1 [X][Y] C. Rate = 0.05 M-2s-1 [X][Y]2 D. Rate = 0.005 M-2s-1 [X][Y]2
Correct answer: A The general form for the rate equation for this reaction is: rate = k[X]a[Y]b, where a and b are the reaction orders for each given reactant. First, determine the reaction orders for the two reactants. Comparing Trials 2 and 3 shows that when [X] is doubled, the reaction rate is also doubled. Therefore, reaction order is first-order with respect to X, and the exponent is 1. Comparing Trials 1 and 2 shows that when [Y] is doubled, rate is also doubled. Therefore, reaction order is first-order with respect to Y, and the exponent is also 1. So far, the expression is rate = k[X]1[Y]1 = k[X][Y]. Now, determine the rate constant k using the data from one of the experimental runs (say, Trial 1): rate = k X Y ⇒ k = rate [ X ] [ Y ] k = 1 . 25 × 10 - 2 0 . 5 2 = 1 . 25 × 10 - 2 0 . 25 = 5 × 10 - 2 M - 1 s - 1 Therefore, (A) is correct.
Oral bacteria can cause tooth enamel decay by destroying a modified version of hydroxyapatite, a compound found in teeth comprised of calcium and phosphorous. Fluoride has been included in toothpaste and some tap water as a public health initiative to both prevent and repair such a decay. Which of the following accurately describes a possible mechanism for this protection? A. Fluorine's high electronegativity enables it to bind electrons tightly and form a strong bond with hydroxyapatite, preventing binding by bacterial secretions B. Fluorine's low electronegativity enables it to form weak transient bonds with hydroxyapatite, allowing it to compete for binding sites with bacterial secretions C. Fluorine's large atomic size enables it block bacterial pores, hindering release of secretions that break down hydroxyapatite D. Fluorine's small atomic size enables it to bind diffusely with hydroxyapatite, leading to further break down that initiates the body's natural repair system
Correct answer: A This question requires knowledge of periodic trends. Since electronegativity increases and atomic radius decreases as we move from left to right across a period and bottom to top of a group, Fluorine has one of the smallest atomic radii and is the most electronegative atom. Only (A) correctly identifies a characteristic of fluoride and provides a mechanism consistent with the question stem. (B) and (C) incorrectly identify a characteristic of fluoride. The question stem states fluoride prevents and repairs decay, not cause further breakdown as stated in (D).
According to the kinetic molecular theory, which of the following is true? I. Increasing the temperature of the gas results in an increase in average speed II. Increasing the temperature of the gas results in a decrease in average kinetic energy III. Gases with larger molar masses move faster because of increased inertia A. I only B. I and II C. I and III D. II, and III
Correct answer: A When answering roman numeral questions, it is best to start by evaluating the answer choice that appears most often. In this case, it is (I). The assumption in the kinetic molecular theory of gases that is relevant to (I) states that the temperature of a gas is proportional to its average kinetic energy as well as to the square of its velocity, given by the equation K E = 1 2 m v 2 = 3 2 k B T . Thus, (I) must be true while (II) must be false, eliminating (B) and (D). Furthermore, recall that the root-mean-square speed of a gas is given by the equation u rms = 3 R T M , which means that the average speed of a gas is inversely proportional to the square root of the molar mass. Thus, (III) is false, allowing (C) to be eliminated and making (A) the correct answer. For reference, the kinetic molecular theory of gases makes the following assumptions: • Gases are made up of particles with volumes that are negligible compared to the container volume. • Gas atoms or molecules exhibit no intermolecular attractions or repulsions. • Gas particles are in continuous, random motion, undergoing collisions with other particles and the container walls. • Collisions between any two gas particles (or between particles and the container walls) are elastic, meaning that there is conservation of both momentum and kinetic energy. • The average kinetic energy of gas particles is proportional to the absolute temperature of the gas (in Kelvins), and it is the same for all gases at a given temperature, irrespective of chemical identity or atomic mass.
The probability of formation of gramicidin channels in a membrane is proportional to which of the following? A. [M] B. [M]2 C. [D] D. [D]2
Correct answer: B According to Paragraph 4 of the passage, the action of gramicidin is a second-order reaction and is given by the reaction 2M ⇌ D. That means that the rate of reaction, or frequency (probability) of formation of channels, must be proportional to [M]2. Match to (B).
The carboxylic acid in histidine has a pKa of 1.77 while the carboxylic acid in phenylalanine has a pKa of 2.58. For equal amounts of phenylalanine and histidine at a given pH: A. more of histidine's carboxylic acid groups will exist in the protonated state. B. more of phenylalanine's carboxylic acid groups will exist in the protonated state. C. the carboxylic acid groups of both amino acids will be equally protonated. D. more of histidine's carboxylic acid groups than phenylalanine's carboxylic acid groups will be protonated at some pH's, and less at other pH's.
Correct answer: B According to the Henderson-Hasselbach equation, the pKa is the pH at which a species exists equally in the protonated and deprotonated form (i.e. [weak acid]=[conjugate base]). So, at a pH of 2.58, a sample of phenylalanine will have half its carboxylic acid protonated and half deprotonated. But the histidine carboxylic acid's pKa of 1.77 is almost a whole log point lower, which means there will be almost 10 times more deprotonated histidine than protonated histidine at a pH of 2.58. In other words, the carboxylic acid of histidine is more acidic than the carboxylic acid of phenylalanine. This difference holds true at all pH's, even physiological pH, making (B) correct. Both amino acids will be mostly deprotonated at physiological pH of 7.4, but histidine will be even more deprotonated than phenylalanine.
Which of the following accurately lists a group of elements that share a common number of valence electrons? A. Titanium, potassium, zinc B. Oxygen, polonium, sulfur C. Barium, strontium, sodium D. Boron, arsenic, fluorine
Correct answer: B Elements with the same number of valence electrons are located in the same group (column) on the period table. Evaluating the answer choices shows us that (B) lists three elements in the same group, and is correct.
In which of the following species does hydrogen have a negative oxidation number? A. HCl B. NaH C. HCO3- D. NH3
Correct answer: B Hydrogen, a Group IA element, will usually have an oxidation state of +1. However, when coupled to a less electronegative element, hydrogen can assume an oxidation state of -1, specifically in metal hydrides, such as NaH, LiH, etc.. Sodium is also a Group IA element, but has a greater fundamental quantum number. According to periodic trends, it is less electronegative. Combining these facts will lead to (B) as the correct answer.
Which of the following relationships regarding acetaminophen (C8H9NO2), a commonly used fever reducer, is correct? A. Molecular mass (in amu) > Molar mass (in grams) B. Molar mass > 96.08 g C. Molar mass (in grams) < Molecular mass (in amu) D. Molecular mass < 96.08 amu
Correct answer: B The molecular mass in atomic mass units per molecule is equivalent to the molar mass in grams/mole. This eliminates (A) and (C). Mass of acetaminophen must be greater than 96.08 g/mol, which is the molar mass of eight carbons by themselves. This matches (B). The mass of one mole of a compound is called its molar mass and is usually expressed in g/mol. The term molecular weight is sometimes used incorrectly to imply molar mass; remember, molecular weight is measured in amu/molecule not g/mol .
What is the purpose of the calcium hydroxide in the hair removal stage of the tanning process? A. Sodium sulfide and calcium hydroxide form a gritty paste when mixed which acts as an exfoliant to remove the hair by scrubbing. B. The sulfide-keratin reaction occurs only under basic conditions, so calcium hydroxide is needed to keep the pH basic. C. Calcium hydroxide is basic, so the hair is softened, making it easier to remove. D. Calcium hydroxide hardens the hair, making it easier to pull off.
Correct answer: B The passage indicates that sulfide can only attack the keratin in the hair properly at alkaline pH. Recall that alkaline pH is the same as basic, so calcium hydroxide is used to keep the pH high. Therefore, (B) is correct. (C) mentions the basicity of the calcium hydroxide solution, but it distorts the meaning of the passage. The keratin is dissolved by the sulfide ion, not the calcium hydroxide itself, so (C) is incorrect. D) is incorrect because hardening the hair is never mentioned in the passage. (A) is incorrect because the passage does not suggest that sodium sulfide and calcium hydroxide both attack the hair physically, but rather that one attacks chemically through the aid of the other.
An activated G-protein subunit has a short lifespan, spontaneously converting GTP back into GDP and inorganic phosphate under all conditions. What is the enthalpy and entropy change for this reaction? A. Positive change in enthalpy, negative change in entropy. B. Negative change in enthalpy, positive change in entropy. C. Negative change in enthalpy, negative change in entropy. D. Positive change in enthalpy, positive change in entropy.
Correct answer: B The question stem states that GTP ⟶ GDP spontaneously under all conditions. Saying that a reaction is spontaneous is equivalent to saying that ΔG, or the Gibbs free energy, is negative. The equation for Gibbs free energy is ΔG = ΔH - TΔS, where ΔH is the change in enthalpy, and ΔS is the change in entropy. If ΔG is negative under all conditions (at all temperatures), that means ΔH is always negative and ΔS is always positive. (B) is correct. Note that we could also arrive at (B) from another direction: It is a truism of biochemistry that ATP ⟶ ADP, or GTP ⟶ GDP, are exothermic reactions (negative ΔH), and whenever an aqueous solute breaks up from one into two pieces, entropy (disorder) must be increasing (positive ΔS), hence ΔG must be negative.
Anemia caused by iron-deficiency can lead to symptoms of fatigue, difficulty breathing, headache, dizziness, pale skin and tingling in the extremities. For iron to be properly stored, it must be bound to an intracellular protein, ferritin, as Fe3+. What might lead to inadequate storage of iron and thus iron-deficiency? A. Iron's high melting point enables it to remain in the body longer, giving it a higher probability of binding to ferritin B. If iron in the form Fe2+ cannot be oxidized to Fe3+, then it will remain unbound to ferritin C. Iron's high electron affinity allows it strip single electrons from oxygen, creating oxygen radicals that damage cellular processes D. Iron's classification as a metal makes it a good conductor of electricity
Correct answer: B The question tells us that iron can only be stored in the body when it is bound to ferratin as Fe3+. Any explanation in which one or both of the two components, Fe3+ or ferratin, are altered would be a probable cause of iron deficiency. Since iron is a transition metal, it is capable of forming multiple oxidation states, inability to oxidize to Fe3+ is a correct characteristic of iron would match (B).
A medical student studying various types of anesthesia was puzzled to find that both argon and xenon have anesthetic properties. The student's confusion stemmed from his assumption that: A. both atoms were too heavy and would sediment in the body before being able to react with any neurological structures or cause therapeutic effects. B. both atoms were inert and not likely to react with any neurological structures or cause therapeutic effects. C. both atoms would be oxidized before being able to react with any neurological structures or cause therapeutic effects. D. the high electron affinity of both atoms would make them too reactive and damage neurological structures.
Correct answer: B Xenon and argon are inert, meaning they do not readily bond with other atoms. However, this does they can still interfere with physiological functions by blocking receptors for excitatory neurotransmitters such as glutamate. While the mechanism for this action is beyond the scope of the MCAT, you can use logic to arrive at the correct answer choice, since (B) is the only choice that correctly describes a characteristic property of noble gases.
This table shows dissociation constants (Kd) between several biological molecules and human serum albumin. Molecule Kd Thyroxine 3.2 × 10-6 Palmitate 6.7 × 10-9 Cortisol 3.3 × 10-4 Which one has the highest affinity for albumin? A. Cortisol B. Thyroxine C. Palmitate D. It cannot be determined from the data in the table
Correct answer: C Affinity is a measurement of how strongly two molecules bind each other. A dissociation constant, on the other hand, expresses the tendency of two molecules which can bind to each other to disassociate--or separate. In other words, if Kd is small, the components of the complex have a high affinity for each other, and if Kd is large, the components have a low affinity for each other. Of the three substances in the table, palmitate has lowest disassociation constant with serum albumin, and thus the highest affinity with it, and (C) is correct.
Boric acid, B(OH)3, is a topical anesthetic in which boron acts as a Lewis acid. However, boron instead behaves as an anion in the superconducting compound, magnesium diboride (MgB2). This can be accounted for by the fact boron is: A. a metal. B. a good conductor. C. a metalloid. D. ductile.
Correct answer: C Boron borders the metalloid line—a stair-step group of elements separating the metals and nonmetals on the periodic table—and is therefore classified as a metalloid or semimetal. These elements share properties of both metals and nonmetals, which enables boron to act as either a Lewis acid or an anion depending on the other atoms with which it is bound. Thus, (C) is correct.
If the ΔH of a biological reaction is found to be positive, what can we conclude about the reaction? A. The reaction will proceed spontaneously. B. The reaction will not proceed spontaneously. C. Heat is absorbed during the reaction. D. Heat is released during the reaction.
Correct answer: C Exothermic Reaction Diagram Most reactions in the lab occur under constant pressure (at 1 atm) in closed thermodynamic systems. To express heat changes at constant pressure, chemists use the term enthalpy (H). Enthalpy is a state function, so we can calculate the change in enthalpy (ΔH) for a system that has undergone a process—for example, a chemical reaction—by comparing the enthalpy of the final state to the enthalpy of the initial state, irrespective of the path taken. The change in enthalpy is equal to the heat transferred into or out of the system at constant pressure. To find the enthalpy change of a reaction, ΔHrxn, one must subtract the enthalpy of the reactants from the enthalpy of the products: ΔH rxn=H products−H reactants As seen in the images above, a positive ΔHrxn corresponds to an endothermic process, and a negative ΔHrxn corresponds to an exothermic process. Endothermic reactions (+ΔH), such as the phase change from ice to water, are associated with an absorption of heat. Therefore, (C) is correct. Exothermic reactions (-ΔH), such as the burning of logs, are associated with a release of heat, making (D) opposite. Gibb's free energy, G, is a combination of enthalpy, entropy, and temperature. The change in Gibbs free energy, ΔG, is a measure of the change in the enthalpy and the change in entropy as a system undergoes a process, and it indicates whether a reaction is spontaneous or nonspontaneous. The change in the free energy is the maximum amount of energy released by a process—occurring at constant temperature and pressure—that is available to perform useful work. The change in Gibbs free energy is defined as follows: ΔG=ΔH−TΔS where T is the temperature in kelvin and TΔS represents the total amount of energy that is absorbed by a system when its entropy increases reversibly. Depending on the change in entropy and temperature, reactions with a +ΔH can be spontaneous or nonspontaneous, eliminating (A) and (B).
The sodium-potassium pump maintains a cell's osmolarity by transporting Na+ and K+ against their concentration gradients. In which of the following ways are these ions most similar? A. Ionic radius B. Number of protons C. Number of valence electrons D. Intracellular concentration
Correct answer: C On the periodic table of elements, sodium (Na) and potassium (K), numbers 11 and 19 respectively, are both located in Group I. Elements in the same group exhibit similar characteristics and behavior because they have the same number of valence electrons. Both Na and K have 1 valence electron (remember: the group will tell you how many valence electrons there are). With only one electron, these elements are prone to losing the electron and becoming cations (positively charged). After losing one electron each, Na+ and K+ continue to have the same number of valence electrons (C).
Which of the following is MOST likely to be found in the salt bridge of a galvanic cell? A. Ar B. CH4 C. MgSO4 D. C6H12O6
Correct answer: C Salt bridges contain inert electrolytes. Of the options, only (C), magnesium sulfate, is a salt that will completely dissociate in solution into the cation and anion form Mg2+ and SO42-, respectively. What is the purpose of a salt bridge? In a galvanic cell, if only a wire were provided for this electron flow, the reaction would soon stop because an excess positive charge would build up on the anode, and an excess negative charge would build up on the cathode. Eventually, the excessive charge accumulation would provide a countervoltage large enough to prevent the oxidation-reduction reaction from taking place, and the current would cease. This charge gradient is dissipated by the presence of a salt bridge, which permits the exchange of cations and anions. The salt bridge contains an inert electrolyte, usually KCl or NH4NO3, which contains ions that will not react with the electrodes or with the ions in solution. While the anions from the salt bridge diffuse into the solution on the anode side to balance out the charge of the newly created cations, the cations of the salt bridge flow into the solution on the cathode side to balance out the charge of the anions left in solution when the cations are reduced and precipitate onto the electrode.
The rate of a reaction increases as temperature increases because: A. the activation energy of the reaction increases. B. the activation energy of the reaction decreases. C. the average kinetic energy of the molecules increases. D. the average kinetic energy of the molecules decreases.
Correct answer: C Since temperature is a measure of average kinetic energy, an increase in temperature increases the average kinetic energy of the molecules. With a higher average kinetic energy, molecules collide more often and have enough energy to achieve the activation energy needed for the reaction, leading to a faster reaction rate. Note that an increase in temperature helps overcome the energy needed for the reaction to proceed, but does not change the activation energy. This matches (C).
The biological affinity of a melatonin receptor to an oxygen-containing drug was studied by a pharmaceutical company. The company then made the same drug, but instead used sulfur in lieu of oxygen. They found that affinity was decreased, but it was still comparable to the oxygen-containing drug because oxygen and sulfur: A. contain a different number of valence electrons. B. contain the same number of total electrons. C. are in the same group of the periodic table. D. are in the same period of the periodic table.
Correct answer: C Sulfur and oxygen both belong to Group 16 (VIa) of the periodic table, which means they would have the same number of valence electrons and, as a result, have similar chemical properties. Due to this fact, they would be expected to behavior similarly in biological systems, making (C) correct.
Which of the steps in the leather tanning process uses the most acidic conditions? A. Preservation/ bacteria removal B. Bating C. Pickling D. Tannage
Correct answer: C This is an excellent example of a question for which an understanding of the passage is necessary. Let's examine each of the answer choices individually to determine whether the process is performed under acidic conditions. The bacteria killing/preservation stage from the beginning of the tanning process is performed with salt, so it is not acidic. Eliminate (A). The bating process uses enzymes, and nothing is said about their pH, so eliminate (B). The pickling process uses a mixture of salt and sulfuric acid, so it is very acidic. Keep (C) and verify that (D) is incorrect: tannage uses tannin for hard leather and chromium (III) sulfate for soft leather, and nothing is said about the pH of these substances, so (C) is correct.
In the reaction between reactants A and B, the rate is doubled when the concentration of A is doubled, and the rate is quartered when the concentration of B is halved. Which of the following is a possible rate law for this reaction? A. Rate = k[A]2[B] B. Rate = k[A][B] C. Rate = k[A][B]2 D. Rate = k[A]2[B]2
Correct answer: C This question provides the change in rate with the change in concentrations and is asking about the rate law. There are three main types of orders of reaction: zero order, first order, and second order. A zero-order reaction is one in which the rate of formation of product C is independent of changes in concentrations of any of the reactants, A and B. These reactions have a constant reaction rate equal to the rate constant (rate coefficient), k. The rate law for a zero-order reaction is: rate = k[A]0[B]0. A first-order reaction has a rate that is directly proportional to only one reactant, such that doubling the concentration of that reactant results in a doubling of the rate of formation of the product. The rate law for a first-order reaction is rate = k[A]1 or rate = k[B]1. A second-order reaction has a rate that is proportional to either the concentrations of two reactants or to the square of the concentration of a single reactant. The following rate laws all reflect second-order reactions: rate = k[A]1[B]1 or rate = k[A]2 or rate = k[B]2. According to the question stem, if [A] is doubled, the rate is doubled, implying that this reaction is a first-order reaction with respect to A. For B, the rate is quartered, or is ¼ of the original, when [B] is halved, or ½ of the original. This implies that this reaction is second-order with respect to B. This means the rate law should be k[A][B]2 and (C) is correct.
Titanium dioxide is used as a coating on glass to form photocatalytic, "self-disinfecting" windows and mirrors to reduce the rate of healthcare associated infections. What is the oxidation state of titanium in these coatings? A. -2 B. 2 C. 4 D. 7
Correct answer: C When assigning oxidation states, remember that in most compounds, the oxidation number of oxygen is -2: two electrons are "taken" by each oxygen. In other words, in titanium dioxide four electrons are deficient from Ti. Remember as well that in a neutral compound such as TiO2, the sum of the oxidation numbers of all the atoms present is zero. To balance the negative oxidation states of the two oxygen atoms, the titanium atom carries an oxidation state of +4, so (C) is correct.
Plants utilize light, carbon dioxide, and water in photosynthesis to make a sugar molecule and oxygen. The creation of the sugar by itself is an unfavorable process. Which of the following choices best describes that, assuming standard conditions? A. The cell is a galvanic cell B. Keq > 1 C. ΔG°> 1 D. Q < Keq
Correct answer: C When ΔG° is positive, the reaction is non-spontaneous. The question stem tells us that the process of creating sugar is not favorable. Therefore, only (C) fits what was described in the question stem. Movement away from the equilibrium position is associated with an increase in Gibbs free energy (ΔG > 0) and is nonspontaneous. This would be characteristic of the reactions described in the question stem. Such a reaction is said to be endergonic, and is shown below: All other answer choices are true for spontaneous reactions. Reactions in galvanic cells are spontaneous (A). The same is true when Keq is greater than 1 (B) and the reaction quotient (Q) is less than Keq (D).
A reaction is observed to be spontaneous and to have both positive entropy and enthalpy values. Which of the following is most likely to be true about this reaction? A. ΔG must be greater than 11 B. The components of the reaction are all in a solid phase C. The temperature of the reaction is 782 Kelvin D. The reaction will make more reactants than products
Correct answer: C +/+ Spontaneous at high T +/- Nonspontaneous at all T -/+ Spontaneous at all T -/- Spontaneous at low T Recall the formula for Gibbs free energy: ΔG=ΔH−TΔS The table above demonstrates the spontaneity of various combinations of enthalpy and entropy. When both entropy and enthalpy are positive, the reaction will only be spontaneous under high temperature conditions (C).
Propane reacts with oxygen to produce carbon dioxide, water, and heat. Which of the following correctly describes the reaction and reaction type? A. C3H8 + O2 ⟶ CO2 + H2O + heat; double-displacement B. C3H8 + O2 ⟶ CO2 + H2O + heat; combustion C. C3H8 + 5O2 ⟶ 3CO2 + 4H2O + heat; double-displacement D. C3H8 + 5O2 ⟶ 3CO2 + 4H2O + heat; combustion
Correct answer: D (D) is the correct answer because it contains the balanced equation and appropriately identifies the reaction as a combustion reaction. One method of balancing the equation would be to balance the carbon first, then the hydrogen, then the oxygen. This is a combustion reaction because a fuel (usually a hydrocarbon, like C3H8) is mixed with an oxidant (usually oxygen), forming carbon dioxide and water.
In the following acid-base reaction: HCO−3+HCl⇌H2CO3+Cl− A. HCl is an acid and HCO3- is its conjugate base. B. Cl- is an acid and HCl is its conjugate base. C. HCO3- is a base and Cl- is its conjugate acid. D. HCO3- is a base and H2CO3 is its conjugate acid.
Correct answer: D Because the Brønsted-Lowry definition of an acid-base reaction is one in which a hydrogen ion (proton) is transferred from an acid to a base, the two always occur in pairs called conjugates. A conjugate acid is the acid formed when a base gains a proton, and a conjugate base is the base formed when an acid loses a proton. For example, HCO3− (aq) + H2O (l) ⇌ CO32− (aq) + H3O+ (aq) CO32− is the conjugate base of HCO3− , a weak acid, and H3O+ is the conjugate acid of H2O, a weak base. Now let's go through these a bit more systematically. (A) states that HCO3- is the conjugate base of HCl. This would mean that if HCO3- gained a proton, it would form HCl, which it does not. It instead forms H2CO3. (B) is also wrong because Cl- is not an acid since it has no proton to donate. HCl is a strong acid, and not a base, as its proton is able to freely dissociate. For (C), HCO3- is a base but its conjugate acid would be the species created when it gains a proton: H2CO3. (D) is the only correct answer, as HCO3- is a base and adding a proton to would create its conjugate acid, H2CO3. A conjugate acid has one more H+ than the compound with which it is being compared. In other words, the conjugate acid loses a proton to become its conjugate base. In the same way, a conjugate base can gain a proton to become its conjugate acid. Bicarbonate is a base that forms carbonic acid upon the addition of a proton. Thus, the correct answer is (D).
Which of the following statements accurately explains why acetic acid has a significantly higher boiling point than acetaldehyde? A. Acetic acid has a greater molecular weight B. Acetaldehyde exhibits more hydrogen bonding C. Acetic acid has weaker intramolecular forces D. Acetic acid has stronger intermolecular forces
Correct answer: D Boiling point is dependent on molecular weight and intermolecular forces. The greater the molecular weight, and the greater the attraction due to intermolecular forces, the higher the boiling point will be. While there is a difference in molecular weight between both molecules, it is relatively minor. The hydrogen bonding in acetic acid is stronger than the the dipole-dipole interactions in acetaldehyde and accounts for the bulk of the difference (D).
Place the following systems in order of increasing entropy I. Liquid at 100 °C II. Solid at 0 °C III. Vapor at 100 °C IV. Liquid at 0 °C A. II < IV < III < I B. IV < II < I < III C. III < I < IV < II D. II < IV < I < III
Correct answer: D Entropy increases as the number of available microstates increases, and the number of available microstates increases based on how "free" the molecules are to move. Really though, we can think of entropy as a measure of how much energy is spread out and how widely dispersed is that energy. Entropy always increases from solid⟶liquid⟶gas phase. Additionally, entropy increases as temperature increases within a phase, since temperature is a measure of the average energy of a system. Therefore, a solid at 0° C will have less entropy than a liquid at 0° C, and a liquid at 100° C will have less entropy than a vapor at 100° C. (D) correctly orders the systems.
Phosphoric acid, H3PO4, can be combined with calcium hydroxide, Ca(OH)2 to produce Water, H2O, and calcium phosphate, Ca3(PO4)2, a precursor of hydroxyapatite found in bone according to the following reaction: 2 H3PO4 + 3 Ca(OH)2 ⟶ Ca3(PO4)2 + 6 H2O What type of reaction is indicated by this conversion? A. Single displacement B. Combustion C. Decomposition D. Double displacement
Correct answer: D In this case, phosphate and hydroxyls are being swapped between two different elements. In double-displacement reactions, elements from two different compounds swap places with each other to form two new compounds, matching (D). This type of reaction occurs when one of the products is removed from the solution as a precipitate or gas or when two of the original species combine to form a weak electrolyte that remains undissociated in solution. A single-displacement reaction (A) occurs when an atom or ion in a compound is re- placed by an atom or ion of another element. For example, solid copper metal will displace silver ions in a clear solution of silver nitrate to form a blue copper sulfate solution and solid silver metal. A combustion reaction (B) is a special type of reaction that involves a fuel—usually a hydrocarbon—and an oxidant (normally oxygen). In its most common form, these reactants form the two products of carbon dioxide and water. A decomposition reaction (C) is the opposite of a combination reaction: a single reactant breaks down into two or more products, usually as a result of heating, high-frequency radiation, or electrolysis.
Which of the following statements concerning periodic table trends is true? A. Those elements with the most metallic character tend to have the smallest atomic radii B. Those elements with the least metallic character tend to be the least electronegative C. As electronegativity increases, first ionization energy decreases D. As atomic radius increases, electron affinity decreases
Correct answer: D Metals are located on the left, so we would expect them to have a larger atomic radius than nonmetals and we can eliminate (A). Electronegativity increases as we move to the right and up, and nonmetals are found on the right of the periodic table, so we can also eliminate (B). Electronegativity measures an atom's affinity for electrons, while ionization energy measures the energy required to remove an electron from an atom. If an atom has a relatively high electronegativity, it also has a relatively large first ionization energy, eliminate (C). First ionization energy, electronegativity, and electron affinity follow the same trend, increasing as we move from left to right and from bottom to top on the periodic table. Atomic radius is opposite, and will decrease as we move from left to right, and from bottom to top on the periodic table, making (D) the correct choice.
The protocol for making a laboratory cleaner calls for 0.25 moles of ammonia. Ammonia is converted to urea in people with healthy liver function but is toxic if the organ has damage. How many grams of ammonia would need to be added to the solution? A. 2.50 × 10-1 g B. 4.26 × 10-1 g C. 2.50 g D. 4.26 g
Correct answer: D The molecular formula for ammonia is NH3, leading to a molecular weight of 17.03 g/mol. 17gmol*0.25mol≈4g In order to obtain 0.25 moles, approximately 4 g of ammonia would be needed, corresponding to (D).
Cu2+ ions are commonly found impurities in fabricated silica fiber optics, and are notorious for their high attenuation of light. Which of the following electronic transition states is most likely occurring in Cu2+ ions after absorbing a photon? A. [Ar]4s33d6 B. [Ar]4s23d8 C. [Ar]3d84p7 D. [Ar]4s13d8
Correct answer: D The question stem states that Cu2+ ions are "notorious for their high attenuation of light," meaning that they must absorb photons, which cause electrons to jump to higher energy states. So first, we must determine the electron configuration for Cu2+, and then think about what an excited electron configuration for this ion would look like. The neutral copper molecule's electron configuration is [Ar]4s13d10 (it is not [Ar]4s23d9 because of the extra stability of filled d subshells, but as we shall see, we needn't know this to answer the question correctly).To get Cu2+ we must now remove two electrons from the highest energy states: first the 4s electron, and then one of the d electrons. This leads to a ground state of [Ar]3d9. If this ion absorbs a photon, an electron from the 3d subshell must go to a higher energy state, and (D), [Ar]4s13d8, is correct. Even if we are not sure precisely what the ground state of the Cu2+ ion is, though—after all, transition metals are a bit weird—we can still see that this ion must have nine valence electrons. Only two of the answer choices depict nine total valence electrons, and (A) places too many electrons (three) into an s subshell, leaving (D) as the only possible correct choice.
If a reaction rate is zero-order with respect to reactant A, how will the reaction rate change when the concentration of A is increased four-fold? A. The relative change in reaction rate will depend on the rate constant k B. Increase by a factor of two C. Increase by a factor of four D. The reaction rate will not change
Correct answer: D Zero-order with respect to reactant A means that the rate of the reaction does not depend on the concentration of A. That means that if the concentration of A is increased by a factor of four, the reaction rate should not change at all. The correct answer is (D).
Zinc pyrithione, a common ingredient in anti-dandruff shampoo, is believed to kill scalp fungus by importing metal ions into the cell. What is the most likely element to be imported? A. Arsenic B. Copper C. Silicon D. Germanium
Correct answer:B The question stem informs us that this shampoo ingredient imports metal ions into the fungal cell. Arsenic, silicon, and germanium are metalloids and not true metals. (B) is the only true metal given, and therefore the correct answer.
