Chemistry Final

The reaction of a weak base, B, with HCl is represented by the equation above. The graph shows the titration curve for 25.0mL of an aqueous solution of B titrated with 0.100M HCl. Based on the graph, which of the following best estimates the initial concentration of the solution of the weak base?

0.20 M

The diagram above represents the melting of H2O(s). A 2.00mole sample of H2O(s) at 0°C0°C melted, producing H2O(l) at 0°C. Based on the diagram, which of the following best describes the amount of heat required for this process and the changes that took place at the molecular level?

12.0kJ of heat was absorbed to overcome some of the hydrogen bonding forces holding the water molecules in fixed positions in the crystalline structure.

In an experiment to determine the specific heat of a metal, a student transferred a sample of the metal that was heated in boiling water into room-temperature water in an insulated cup. The student recorded the temperature of the water after thermal equilibrium was reached.

1640 J

An equimolar mixture of N2(g) and Ar(g) is kept inside a rigid container at a constant temperature of 300 K. The initial partial pressure of Ar in the mixture is 0.75atm. An additional amount of Ar was added to the container, enough to double the number of moles of Ar gas in the mixture. Assuming ideal behavior, what is the final pressure of the gas mixture after the addition of the Ar gas?

2.25 atm, because doubling the number of moles of Ar doubles its partial pressure.(Under constant V and T, doubling the number of moles of Ar will double its partial pressure to 1.50atm. Because the number of moles of N2 did not change, its initial and final partial pressures are the same (0.75atm because the initial mixture had the same number of moles of each gas). The total pressure is (1.50+0.75)atm or 2.25atm.)

A student prepares a lactic acid-sodium lactate buffer solution by mixing 40.mL of 0.50MHC3H5O3(aq) with 200.mL of 1.0MNaC3H5O3(aq). The pKa of HC3H5O3 is 3.08. What is the pHof the resulting solution?

4.08

According to the information about the dissolution of Ba(IO3)2(s) shown above, the correct value of S, the molar solubility of Ba(IO3)2(s) can be calculated using with of the following mathematical relationships?

4S3=4×10−9 M

A student mixes 50mL of 1.0MHCl and 50mL of 1.0MNaOH in a coffee-cup calorimeter and observes the change in temperature until the mixture reaches thermal equilibrium. The initial and final temperatures (°C) of the mixture are shown in the diagram above of the laboratory setup.

5.5ºC

The diagram above shows the apparatus used for the electrolysis of CuCl2(aq) according to the chemical equation given. Current required to produce 0.125 mol of Cu in 1 hour

A, I=(2×0.125×96,485 / 3,600)A

The diagram above shows the distribution of speeds for a sample of N2(g) at 25°C. Which of the following graphs shows the distribution of speeds for a sample of O2(g) at 25°C (dashed line) ?

A, higher diagram

In pure water, some of the molecules ionize according to the equation H2O→H++OH−. The extent of the ionization increases with temperature. The graph above shows the pH of pure water at different temperatures. Which of the following represents the variations in the pOH of pure water under the same conditions?

A, identical graph to pH

Which of the following equations can be used to correctly calculate the pH of a solution at 25°C that is 0.100M(CH3)2NH(aq) and 0.100M(CH3)2NH2Cl(aq)?

A, pH = -log (1.0x10-14/5.4x10-4)

A gas mixture at 0°C and 1.0atm contains 0.010mol of H2, 0.015mol of O2, and 0.025molof N2. Assuming ideal behavior, what is the partial pressure of hydrogen gas (H2) in the mixture?

About 0.20atm, because H2 comprises 20% of the total number of moles of gas.

A student mixes 20.0g20.0g of white KCl crystals with distilled water in a beaker. After the mixture was stirred, no crystals are visible and the solution is clear. After several days, all of the water evaporates and white crystals are found in the beaker. Which of following pieces of experimental evidence would best help the student to confirm that a new compound had not been made and that only a physical change occurred?

After the water has evaporated, the white crystals in the beaker have a mass of 20.0g.

For a classroom demonstration, a chemistry teacher puts samples of two different pure solid powders in a beaker. The teacher places the beaker on a small wooden board with a wet surface, then stirs the contents of the beaker.

An endothermic chemical change occurred because the temperature of the beaker and the water on the board decreased as heat was absorbed by the reaction.

A piece of Fe(s) at 25°C is placed into H2O(l) at 75°C in an insulated container. A student predicts that when thermal equilibrium is reached, the Fe atoms, being more massive than the H2O molecules, will have a higher average kinetic energy than the H2O molecules. Which of the following best explains why the student's prediction is incorrect?

At thermal equilibrium, the average kinetic energy of the FeFe atoms cannot be greater than that of the H2OH2O molecules; the average kinetic energies must be the same according to the definition of thermal equilibrium.

the mass spectrum of the element Sb?

B, two lines

Based on the chemical equation and ΔG° given above, which of the following justifies the claim that HA(aq) is a weak acid?

Because ΔG°>>0, Ka<<1, and HA only partially dissociates.

Lewis diagrams of the conjugate acids of some weak nitrogenous bases.

CH3NH2

Each particle diagram shown is a representation of an aqueous solution of one of the acids listed in the table.

Diagram 2 corresponds to HIO.

For the dissolution of HCl in water represented above, which of the following pairs includes the Brønsted-Lowry bases?

H20 and Cl-

vanadium lead galvanic cell, which relationship is used for determining change in mass of the V electrode after 1 minute of the cell's operation without using a balance?

I = q/t

The diagram above is a molecular model of a gaseous diatomic element that is just above its boiling point. Intermolecular forces between the gas molecules will cause them to condense into the liquid phase if the temperature is lowered. Which of the following best describes how the model is limited in its depiction of the phenomenon?

It does not show how the temporary fluctuating dipoles of the molecular electron clouds result in a net force of attraction between the molecules.

The diagram above illustrates how the reaction occurs on the surface of Ru. What is the use of Ru for the synthesis of NH3?

It provides a reaction path with a lower AE

The energy distribution profile (Curve C) for the Y2Y2 molecules is shown in the graph above. Line A represents the most probable energy of the Y2Y2 molecules, and Line B represents the activation energy.

Line B will move to the left because a larger fraction of the Y2 molecules will have the minimum energy to overcome the activation energy barrier.

The table above summarizes data given to a student to evaluate the type of change that took place when substance X was mixed with water. The student claimed that the data did not provide enough evidence to determine whether a chemical or physical change took place and that additional tests were needed. Which of the following identifies the best way to gather evidence to support the type of change that occurred when water and X were mixed?

Measuring the electrical conductivities of XX and the mixture of water and X

the mass spectrum of a sample of a pure element is shown above. based on the data, the peak at 26 amu represents an isotope of which of the following elements?

Mg with 14 neutrons

Which of the following elements has the mass spectrum represented above?

Mo

Which of the following best helps explain why the pressure of a sample of CH4(g) (molar mass 16g/mol) is closer to the pressure predicted by the ideal gas law than a sample of NH3(g) (molar mass 17g/mol) ?

NH3 molecules are polar while CH4 molecules are not, and the greater attractions between NH3 molecules cause the molecules to collide with the walls of the container with less force.

Which of the following phase changes involves the transfer of heat from the surroundings to the system?

NH3(l)→NH3(g), because NH3 molecules in the liquid phase must absorb energy in order to overcome their intermolecular attractions and become free gas molecules.

When 0.20 M NH4Cl(aq) and 0.20 M NaOH(aq) are mixed, the reaction represented by the equation above occurs and a strong smell of ammonia, NH3, is observed. Based on this information, which of the following statements is true?

NH4+(aq) is a stronger acid than H2O(l) is.

The proposed rate-determining step for a reaction is 2 NO2(g)→NO3(g)+NO(g)2. The graph above shows the distribution of energies for NO2(g) molecules at two temperatures. Based on the graph, which of the following statements best explains why the rates of disappearance of NO2(g) are different at temperature 2 and temperature 1 ?

NO2(g) is consumed at a faster rate at temperature 2 because more molecules possess energies at or above the minimum energy required for a collision to lead to a reaction compared to temperature 1.

The decomposition of HI(g) at 298K is represented by the equilibrium equation above. When 100.torr of HI(g) is added to a previously evacuated, rigid container and allowed to reach equilibrium, the partial pressure of I2(g) is approximately 3.7 torr If the initial pressure of HI(g) is increased to 200. torr and the process is repeated at the same temperature, which of the following correctly predicts the equilibrium partial pressure of I2(g), and why?

PI2≈7.4 torr, because it is directly proportional to the initial pressure of HI.

The initial rates of the reaction represented by the equation shown above were measured for different initial concentrations of NO(g) and Cl2(g). Based on the data given in the table above, which of the following is the rate law expression for the reaction, and why?

Rate=k[NO]2[Cl2], because the initial rate quadrupled when [NO] was doubled and doubled when [Cl2] was doubled.

A student adds 50.0g of liquid water at 25.0°C to an insulated container fitted with a temperature probe. The student then adds 10.0g of ice at 0.0°C, closes the container, and measures the temperature at different intervals.

The H2O molecules initially in the ice and the molecules initially in the liquid will have the same average kinetic energy.

The chemical equation above represents the exothermic reaction of CH4(g) with O2(g). Which of the following best helps to explain why the reaction is thermodynamically favored (ΔG<0) at 2000K and 1atm?

The amount of energy released when the product bonds form is much greater than the amount of energy needed to break the reactant bonds.

The reaction is carried out at constant temperature inside a rigid container. Based on this mechanism, which of the following is the most likely reason for the different rates of step 1 and step 2 ?

The amount of energy required for a successful collision between H2 and ICl is greater than the amount of energy required for a successful collision between HI and ICl.

A 1L sample of helium gas at 25°C and 1atm is combined with a 1L sample of neon gas at 25°C and 1atm.

The average kinetic energy of the helium atoms and neon atoms do not change when the gases are combined.

Two students prepared aqueous solutions of LiCl and measured the properties, as shown in the table above. Both students observed that the solid LiCl readily dissolved in H2O.

The better particle diagram was drawn by Student 1 because when LiCl dissolves in water, it dissociates into Li+ and Cl− ions causing an increase in entropy.

buffer capacity: actual volume used is 76.00mL instead of 75.00mL. Based on the error, which of the following is true about the buffer prepared by the student?

The buffer solution will have a slightly higher capacity for the addition of acids than for the addition of bases because the amount of C2H3O2− added was higher than the amount of HC2H3O added.

HF. To prepare a buffer with a pH=3.50, a student needs to mix 250.mL of 0.100MHF0 and 250.mL of 0.100M KF. If the student mistakenly mixes 250.mL of 0.0500M HF and 250.mL of 0.0500M KF, which of the following is the result of this error?

The buffer will have a lower capacity because of the smaller number of moles of HF and F− available to react if an acid or base is added.

The cell potential for the standard galvanic cell shown above is +1.56V. If AgNO3(aq) is replaced with 1MPb(NO3)2(aq) solution and a Pb electrode, which of the following describes what happens to the operation of the cell, and why?

The cell potential decreases because the reduction of Pb2+ is less thermodynamically favorable than the reduction of Ag+

The particle models shown above represent a proposed two-step mechanism for the destruction of ozone (O3) in the upper atmosphere.

The concentration is very low for the duration of the reaction.

Based on the data, which of the following provides the best reason for the trend in base strengths?

The different electronegativities of H, I, and Br

A sample of CHCl3(s) was exposed to a constant source of heat for a period of time. The graph above shows the change in the temperature of the sample as heat is added. Which of the following best describes what occurs at the particle level that makes segment D longer than segment B?

The enthalpy of vaporization is greater than the enthalpy of fusion, because separating molecules completely from the liquid to form a gas requires more energy than separating molecules from their bound crystalline state to a liquid state.

In the spring, blossoms on cherry trees can be damaged when temperatures fall below −2°C. When the forecast calls for air temperatures to be below −5°C for a few hours one night, a farmer sprays his blossoming cherry trees with water, claiming that the blossoms will be protected by the water as it freezes.

The freezing of water is an exothermic process; thus, water that freezes on the blossoms releases heat to keep the blossoms at or above −2°C.

The diagrams above represent two samples of Xe gas in containers of equal volume at 280K. Which of the following correctly compares the two samples in terms of their deviation from ideal gas behavior and explains why?

The gas in sample 2 would deviate more from ideal behavior because the Xe atoms are closer together, leading to an increase in intermolecular attractions.

A student is given two 10g samples, each a mixture of only NaCl(s) and KCl(s) but in different proportions. Which of the following pieces of information could be used to determine which mixture has the higher proportion of KCl(s)?

The mass of Cl in each mixture

A 5.0g sample of MgCl2 may contain measurable amounts of other compounds as impurities. Which of the following quantities is (are) needed to determine that the sample is pure MgCl2 ?

The mass of Mg and the mass of Cl in the sample

A student places a sample of a pure metal in a crucible and heats it strongly in air. Data from the experiment are given in the table above. The final mass was determined after the sample was cooled to room temperature. Which of the following statements related to the experiment is correct?

The mass of the sample increased, so a chemical change occurred when bonds formed between the metal and another substance.

A saturated aqueous solution of CdF2 is prepared. The equilibrium in the solution is represented above. In the solution, [Cd2+]eq=0.0585M and [F−]eq=0.117M. Some 0.90MNaF is added to the saturated solution. Which of the following identifies the molar solubility of CdF2 in pure water and explains the effect that the addition of NaF has on this solubility?

The molar solubility of CdF2 in pure water is 0.0585M, and adding NaF decreases this solubility because the equilibrium shifts to favor the precipitation of some CdF2.

The hydrolysis of sucrose is represented by the chemical equation above.

The reaction proceeds through a different reaction path with a lower AE, intermediate.

In an experiment, X(g) and Y(g) were combined in a rigid container at constant temperature and allowed to react as shown in the equation above. The table provides the data collected during the experiment. Based on the data, which of the following claims is most likely correct?

The reaction reached equilibrium between 75 minutes and 155 minutes after the reactants were combined because the concentrations of X and XY remained constant.

Four different liquid compounds in flasks at 20°C20°C are represented above. The table below identifies the compounds. Flask C shows the most particles in the vapor phase. Which of the following is not shown in the model but best helps to explain why flask C must contain pentane?

The strength of the intermolecular forces between the particles in the liquids

The diagrams above show the ultraviolet absorption spectra for two compounds.

The student forgot to calibrate the spectrophotometer first by using a cuvette containing only acetone.

Which of the following identifies a limitation in how the representations above describe a system of coupled reactions?

The values of ΔG° for each reaction are not shown.

The oxidation of Fe(s) is represented by the chemical equation above. Which of the following correctly explains whether or not the reaction is thermodynamically favorable?

There are more particles (including particles in the gas state) in the reactants than in the product, thus ΔSrxn<0. Because ΔH is large and negative, the reaction will be thermodynamically favorable at low temperatures.

Which of the following best helps to explain why the electron affinity of BrBr has a greater magnitude than that of I?

There is a greater attraction between an added electron and the nucleus in Br than in I.

The student made the standard curve above. Which of the following most likely caused the error in the point the student plotted at 0.050MCo2+(aq)0.050?

There was distilled water in the cuvette when the student put the standard solution in it.

If the experiment is repeated at a higher temperature at which Kc is larger, which of the following best describes the effect of the temperature change on the concentrations of the gases at equilibrium?

There will be a decrease in [A]eq that will be two times the decrease in [B]eq because A and B react in a 2-to-1 ratio.

Two different ionic compounds each contain only copper and chlorine. Both compounds are powders, one white and one brown. An elemental analysis is performed on each powder. Which of the following questions about the compounds is most likely to be answered by the results of the analysis?

What is the formula unit of each compound?

Which of the following best helps explain why the electronegativity of Cl is less than that of F?

When Cl and F form bonds with other atoms, the Cl bonding electrons are more shielded from the positive Cl nucleus than the F bonding electrons are shielded from the positive F nucleus.

The graphs above show Maxwell-Boltzmann distributions for one-mole samples of Ar(g).

When the samples are combined, the gas particles will collide with one another, with the net effect being that energy will be transferred from the more energetic particles to the less energetic particles until a new distribution of energies is achieved at a temperature between 300K and 600K.

A student has two samples of NaCl, each one from a different source. Assume that the only potential contaminant in each sample is KCl. The student runs an experiment to determine the percent by mass of chlorine in each sample. From the results of this experiment alone, which of the following questions is most likely to be answered?

Which sample has the higher purity?

During the day, solar radiation is absorbed by NO3(g),resulting in its decomposition. Which of the following best explains whether the equilibrium concentration of N2O5(g) in the atmosphere in the daytime is different from that in the nighttime, and why?

[N2O5] will be higher at night, because the decomposition of NO3(g) in the daytime will result in an increase in the rate of consumption of N2O5(g) to reform NO3(g).

The diagram above represents the equilibrium between the two isomers of C2H2Cl2, and the table provides the data collected in an experiment to determine its equilibrium constant, Kc, at 490K. In a second experiment done at the same temperature, [Z]eq≈1.0 M.

[Y]eq≈1.5 M because the ratio [Z]eq[Y]eq should remain constant when the reaction is done at the same temperature.

A galvanic cell generates a cell potential of +0.32V+0.32V when operated under standard conditions according to the reaction above. Which of the following pairs of conditions are needed to construct a similar cell that generates the lowest cell potential?

[Zn2+]=2M and [Fe2+]=0.5M because Q>1.

Which of the following represents an electron configuration that corresponds to the valence electrons of an element for which there is an especially large jump between the second and third ionization energies? (Note: n represents a principal quantum number equal to or greater than 2.)

ns2

In pure water, some of the molecules ionize according to the equation H2O→H++OH−The extent of the ionization increases with temperature. A student heats pure water and records the measured pH at 50°C as 6.6. Based on this information, which of the following mathematical relationships gives the pOH of pure water at 50°C

pH = 14 -pOH

In pure water, some of the molecules ionize according to the equation H2O→H++OH−. The extent of the ionization increases with temperature. A student heats pure water and records the measured pH at 50°C50°C as 6.6. Based on this information, which of the following mathematical relationships gives the pOH of pure water at 50°C?

pOH = pH

A 40.0mL sample of the weak base C5H11N was titrated with 1.00M HCl. Based on the resulting titration curve shown above, which of the following pairs provide the best estimates for the pKb and Kb of C5H11N?

pkb = 3.0 and Kb = 1 x 10^-3

Ar(g) deviates more from ideal behavior at extremely high pressures than Ne(g) does. Which of the following is one reason for this difference?

the particle volume of Ar is greater than that of Ne

The synthesis of NH3NH3 is represented by the equation above. Based on the equilibrium constant, K, and ΔH°rxn given above, which of the following can best be used to justify that the reaction is thermodynamically favorable at 298K and constant pressure?

ΔG°=−RTlnK<0 because K>>1

The oxidation of carbon monoxide can be represented by the chemical equation 2 CO(g)+O2(g)→2 CO2(g)2. The table above provides the average bond enthalpies for different bond types. Based on the information in the table, which of the following mathematical expressions is correct for the estimated enthalpy change for the reaction?

ΔHrxn= [2(1072kJmol)+(498kJmol)]−4(799kJmol)

A 2.00mol sample of C2H5OH undergoes the phase transition illustrated in the diagram above. The molar enthalpy of vaporization, ΔHvap, of C2H5OHC2H5OH is +38.6kJ/mol. Which of the following best identifies the change in enthalpy in the phase transition shown in the diagram?

−77.2kJ

A cylinder with a moveable piston is completely filled with a small amount (100 millimoles) of liquid water at a pressure of 1.0atm and a temperature of 80∘C.

Region B, because the pressure inside the cylinder is equal to the vapor pressure of water at 80∘C when both liquid and gas phases are present.

Using a spectrophotometer, a student measures the absorbance of four solutions of CuSO4 at a given wavelength. The collected data is given in the table above. Which of the following is the most likely explanation for the discrepant data in trial 4 ?

The concentration of the solution was actually lower than 0.150 M

The graph above shows the distribution of molecular speeds for four different gases at the same temperature. What property of the different gases can be correctly ranked using information from the graph, and why?

The molecular masses of the gases, because the gas molecules have the same average kinetic energy and mass can be calculated using the equation KEavg=1/2mv.

HF to prepare a buffer with pH≈3.50, 4.20g of NaF(s) should be added to 500.0mL of 0.100M HF(aq). The buffer is accidentally prepared using 90% pure NaF(s) instead of 99% pure NaF(s).

The pH is slightly lower than 3.50 and it has a lower capacity for the addition of acids because less than 4.20g of NaF(s) was added.

Diagram 1 above shows equimolar samples of two gases inside a container fitted with a removable barrier placed so that each gas occupies the same volume?

The partial pressure of each gas in the mixture is half its initial pressure; the final total pressure is half the sum of the initial pressure of the two gases.(For each gas, the partial pressure in the mixture is half its initial pressure because the volume occupied has doubled under constant n and T

Ar(g) deviates more from ideal behavior at extremely high pressures than Ne(g) does. Which of the following is one reason for this difference? Responses

The particle volume of Ar is greater than that of Ne.

Shown above is the chemical equation for the dissolution of the slightly soluble salt CuBr(s). Its Ksp value in pure water was experimentally determined. CuBr was found to be much less soluble in a 0.001M NaBr solution than in pure water. Which of the following correctly explains the decrease in solubility of CuBr in 0.001 M NaBr?

The presence of additional Br− ions already in the solution means equilibrium will be reached when much less CuBr has dissolved.

The galvanic cell shown above generates a cell potential of +3.17V when operated under standard conditions. A second galvanic cell is made from the same two metals, and the measured cell potential is +3.25V. Which of the following could be the reason for the second cell having a greater cell potential?

[Ag+]>1M and [Mg2+]=1M in the second cell, resulting in Q<1, and Q=1 for the first cell.