AP CHEMISTRY Scoring Guide

Unit 7 Progress Check: MCQ

1.

Time (min) Pressure ( )

0.25 101
0.50 108
0.75 116
1.00 124
1.25 128
1.50 130
1.75 130
2.00 130

A sample of acetone is placed into a container. The container is sealed and attached to a pressure sensor, as shown
in the diagram above. The container is allowed to sit on the lab table for a few minutes as the pressure in the
container is monitored at regular intervals. At the end of 2.00 minutes, some acetone liquid remains in the container.
Which of the following best explains the pressure data presented in the table above?
(A) The acetone heats up over time, causing more of it to vaporize at an increasing rate.
(B) The acetone has completely vaporized after 1.50 minutes, so the pressure becomes constant.
The acetone vaporizes from the liquid at a constant rate, the rate of condensation increases until it
(C)
becomes equal to the rate of evaporation, and then the pressure stays constant.
The acetone vaporizes from the liquid at a rate that is fast in the beginning but then slows down until the
(D)
vaporization process stops completely.

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Unit 7 Progress Check: MCQ

Answer C

Correct. A constant equilibrium pressure is achieved once the rate of condensation of the acetone vapor
matches the rate of evaporation of the acetone from the liquid.

2.

A sample of pure in a sealed tube at is placed in a temperature bath at . Observations of

changes in the color, pressure, and mass of the mixture are recorded as a function of time. Which of the following is
an observation that would best support the claim that the reaction represented above has reached equilibrium at
?
The total mass of the system remains constant because the amounts of reactant and product do not
(A)
change with time at equilibrium.
The color of the system changes from brown to completely colorless because only the product will be
(B)
present at equilibrium.
The total pressure of the system decreases then reaches a constant value because the amounts of reactant
(C)
and product no longer change at equilibrium.
The temperature of the system remains constant because the temperature must be constant at
(D)
equilibrium.

Answer C

Correct. As the reaction proceeds to the right, two molecules are converted into one
molecule, resulting in a decrease in the number of molecules in a fixed volume and a decrease in
pressure. At equilibrium, the pressure will be constant since the number of reactant and product
molecules will be constant.

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Unit 7 Progress Check: MCQ

3.

The graph above represents the data collected under certain conditions for the decomposition of
according to the chemical equation above. Based on the graph, at approximately which time is equilibrium
established?
(A) At time , because is expanding to fill the container.
(B) At time , because the reaction is reversible and .
(C) At time , because the reaction is about to reach completion and .

(D) At time , because there are no observable changes in and .

Answer D

Correct. At equilibrium, is transformed into at the same rate at which is transformed

into , and no changes in and are observed.

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Unit 7 Progress Check: MCQ

4.

When is placed in water, it dissolves according to the chemical equation above. The particle diagram
above was proposed to represent an aqueous solution in which is in equilibrium with its ions. Which of
the following best explains whether or not the diagram provides a good representation of this dynamic equilibrium
at the microscopic level?
It is a good representation because it shows that the dipoles of the molecules are oriented around
(A)
the and ions in solution.
It is a good representation because it shows that the concentrations of ions and ions are equal
(B)
at equilibrium.
It is not a good representation because it does not show that the concentration of is constant at
(C)
equilibrium.
It is not a good representation because it does not illustrate the dynamic equilibrium in which the rates of
(D)
the forward and reverse reactions are equal.

Answer D

Correct. Since at equilibrium the rates of the forward and reverse reactions are the same, one single
representation cannot capture the dynamic nature of this process. Multiple representations of the reaction
before and after equilibrium would be needed to show that the concentrations of the ions have remained

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Unit 7 Progress Check: MCQ

constant over time.

5.

The particle diagram above shows the system represented by the equation . Which
of the following explains whether the particle diagram indicates that the system is at equilibrium?
The particle diagram does not indicate that the system is at equilibrium because it shows the system only
(A)
at one point in time.

The particle diagram does not indicate that the system is at equilibrium because the ratio is not
(B)
equal to 1.
(C) The particle diagram indicates that the system is at equilibrium because the value of is small.
(D) The particle diagram indicates that the system is at equilibrium because .

Answer A

Correct. Equilibrium is a dynamic process in which the rates of the forward reaction and the reverse
reaction are equal and the concentrations of reactants and products remain constant over time. A single
representation cannot capture this dynamic process.

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 Scoring Guide

Unit 7 Progress Check: MCQ

6.

The particle diagrams above show the changes that occurred after an equimolecular mixture of and was
placed inside a rigid container at constant temperature. Which of the following statements is be best supported by
the particle diagrams?
(A) The rate of the reverse reaction is zero because the concentration of keeps increasing.
The reverse reaction has a higher rate than the forward reaction between and because
(B)
.
The forward reaction has a faster rate than the reverse reaction between and because more
(C)
products were being formed.
The rates of the forward and reverse reactions were the same after because the rate of formation
(D)
of was constant.

Answer C

Correct. Based on the particle diagrams, the amount of kept increasing between and ,
indicating that the forward reaction occurred at a faster rate than the reverse reaction.

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Unit 7 Progress Check: MCQ

7.

Substance Initial Concentration ( ) Equilibrium Concentration ( )

0.30 0.24
0.00 0.030
0.00 ?

In an experiment involving the reaction shown above, a sample of pure was placed inside a rigid container at a
certain temperature. The table above provides the initial and equilibrium concentrations for some of the substances
in the reaction. Based on the data, which of the following is the value of the equilibrium constant ( ) for the
reaction, and why?
(A) , because .

(B) , because .

(C) , because .

(D) , because .

Answer C

Correct. The mathematical expression for the equilibrium constant is . Since and

are both products and have the same stoichiometric coefficient, their equilibrium concentrations will be
the same. Therefore, .

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Unit 7 Progress Check: MCQ

8.

reacts with to produce , according to the equation above. A mixture of of

and of is placed in a rigid, evacuated container, and the reaction mixture is
allowed to reach equilibrium at a certain temperature. The table below shows the initial partial pressure of each gas
and the equilibrium partial pressures of and at the same temperature.

Initial pressure 1.00 1.00 0.00

Equilibrium pressure 0.20 0.80

Which of the following indicates the equilibrium value of and the approximate value of the equilibrium
constant, ?
(A) and
(B) and

(C) and

(D) and

Answer C

Correct. In the balanced equation for the reaction, the coefficients of and are both 1. Therefore,
the number of atmospheres of that reacted must be the same as the number of atmospheres of
that reacted. Since the equilibrium partial pressure of is , the equilibrium partial pressure
of must be . For the reaction, ; thus, the value of
.

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Unit 7 Progress Check: MCQ

9.

The diagram above represents the equilibrium between two isomers of 2-butene. The equilibrium constant, , is
1.2 at a certain temperature. Two identical vessels each contain an equilibrium mixture of the two gases at that
temperature. The concentration of cis‑2‑butene in the second vessel is twice the concentration in the first vessel.
What is the concentration of trans‑2‑butene in the second vessel compared to that in the first vessel?
(A) Half the concentration of that in the first vessel
(B) The same concentration as that in the first vessel
(C) Twice the concentration of that in the first vessel
(D) Four times the concentration of that in the first vessel

Answer C

Correct. . is constant at constant temperature, so if doubles,

must double.

10.

The equilibrium reaction in is represented by the equation above. Based on the magnitude of
the equilibrium constant, which of the following correctly compares the equilibrium concentrations of substances
involved in the reaction, and why?
The equilibrium concentration of will be much smaller than the equilibrium concentration of
(A)
, because is the solvent and is present in the largest amount.
The equilibrium concentration of will be much smaller than the equilibrium concentration of
(B)
, because .
The equilibrium concentration of will be much smaller than the equilibrium concentration of
(C)
, because all the will react to produce .
The equilibrium concentration of will be much larger than the equilibrium concentration of
(D)
, because .

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Unit 7 Progress Check: MCQ

Answer B

Correct. Because and , .

11.

At a certain temperature, and react to produce according to the chemical equation shown
above. An evacuated rigid vessel is originally filled with and , each with a partial pressure of
. Which of the following is closest to the partial pressure of after the system has reached equilibrium, and
why?
; because is very large, nearly all the and are consumed before the system
(A)
reaches equilibrium.
; because is very large, nearly all the is consumed before the system reaches
(B)
equilibrium, but an excess amount of remains at equilibrium.
; because is very large, the system is already near equilibrium, and there will be very little
(C)
change to the partial pressure of .
; because is very large, the decomposition of any that forms increases the amount
(D)
of at equilibrium.

Answer B

Correct. Based on the large , the reaction will proceed almost to completion, but based on the
stoichiometry, is a limiting reactant; thus, the excess will have partial pressure
approximately equal to .

12.

A sample of solid zinc is added to of . After the mixture sits

overnight, which of the following best describes what will most likely be observed and measured the next morning
and why?
Almost all of the will still be in the beaker with no visible , because the initial mixture is
(A)
already at equilibrium due to the very large .
About half of the will have disappeared and will have appeared in the beaker, because the
(B)
system reaches equilibrium.
About two-thirds of the will have disappeared and will have appeared in the beaker,
(C)
because the system reaches equilibrium.
Virtually all of the will have disappeared and will have appeared in the beaker, because
(D)
the reaction proceeds almost to completion at equilibrium due to the very large .

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Unit 7 Progress Check: MCQ

Answer D

Correct. The reaction should proceed almost to completion at equilibrium because is very large,
resulting in a yield of about of and a negligible amount of that would not be
visibly detectable.

13.
Reaction Chemical Equation
1
2

The table above provides the values for two reactions. Which of the following is the correct mathematical
expression needed to determine the equilibrium constant of the reaction shown below?

Reaction 3:

(A)

(B)
(C)
(D)

Answer A

Correct. Reaction 2 needs to be reversed to obtain reaction 3. When a reaction is reversed, the
equilibrium constant for the reverse reaction is the reciprocal of the equilibrium constant of the original
reaction. When reactions are added together, the of the resulting overall reaction is the product of the
’s for the reactions that were summed. Therefore, .

14.
Equation 1:

Equation 2:

In a large reaction vessel at a constant temperature, nitrogen dioxide and dinitrogen tetroxide are in a state of
dynamic equilibrium, as represented by the chemical equations shown above. The equilibrium constants for the
reactions are and . Which of the following quantities can most easily be used to find the value of ?

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Unit 7 Progress Check: MCQ

(A) The value of for the reaction

(B) The temperature of the system
(C) The volume of the system
(D) The value of

Answer D

Correct. If the value of is known, then the relationship can be used to determine the value
of .

15.
Reaction 1:

Reaction 2:

Based on the information given above, which of the following is the expression for for the reaction that occurs
when a is added to a saturated solution of , as represented by the following
chemical equation?

(A)
(B)
(C)

(D)

Answer D

Correct. To obtain the net equation, the equation 2 must be reversed and doubled; thus, is
determined by multiplying by the reciprocal of squared.

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Unit 7 Progress Check: MCQ

16.

The formation of in an aqueous solution is represented by the chemical equilibrium shown above. A
light red-brown solution is prepared combining of , of ,
and of water that had been slightly acidified. If an additional of is added,
which of the following predicts and explains correctly whether the darkness of the color of the solution will change?
(A) The color of the solution will lighten because most of the volume added is solvent.
(B) The color of the solution will darken because the equilibrium will favor the dissociation of .
The color of the solution will darken because the equilibrium will favor the formation of more
(C)
.
(D) The color of the solution will not change because the solution already contains ions.

Answer C

Correct. Increasing the amount of a reactant in a solution at equilibrium will favor the formation of
products. In this case, the addition of ions disrupts the equilibrium. As a result, the reaction favors
the formation of , and the solution will darken (will turn a darker shade of red-brown).

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Unit 7 Progress Check: MCQ

17.

The reaction of with is represented by the balanced equation above.

A chemist carried out the reaction at , starting with of each reactant in a container with
variable volume. The reaction mixture quickly reached equilibrium, as indicated in the graph above. After 20
seconds, the chemist reduces the volume of the equilibrium system by half while keeping the temperature constant.
Which of the following predictions about the yield of is best, and why?
By halving the volume, the pressure doubles. The system will respond to the increase in pressure by
(A) decreasing the total number of moles of gas in the system. Thus, the yield will increase because the
reaction will shift toward more product.
By halving the volume, the pressure doubles. The system will respond to the increase in pressure by
(B) increasing the total number of moles of gas in the system. Thus, the yield will decrease because the
reaction will shift toward more reactants.
By halving the volume, the pressure is reduced by half. The system will respond to the decrease in
(C) pressure by increasing the total number of moles of gas in the system. Thus, the yield will decrease
because the reaction will shift toward more reactants.
By halving the volume, the pressure is reduced by half. The system will respond to the decrease in
(D) pressure by decreasing the total number of moles of gas in the system. Thus, the yield will increase
because the reaction will shift toward more products.

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Unit 7 Progress Check: MCQ

18.

A student poured of into a test tube and added a few drops of concentrated , which
resulted in a deep-blue solution. The reaction that occurred is represented by the chemical equation shown above.
Then, the student placed the test tube inside a beaker that contained ice and water for about five minutes. Which of
the following describes what the student most likely observed next, and why?
The color of the solution changed from blue to pink, because lowering the temperature increased the
(A)
collision frequency between and .
The color of the solution changed from blue to pink, because cooling caused the equilibrium to shift to
(B)
form the pink-colored .
(C) The color of the solution did not change, because more water was not added to the solution.
(D) The color of the solution did not change, because heat is not released from this reaction.

Answer B

Correct. The forward reaction is endothermic; thus, cooling the test tube and its contents shifts the
equilibrium toward the formation of the pink-colored .

19.
Substance Concentration
0.10
0.30
0.20

The chemical reaction shown above took place inside a rigid container at constant temperature. The table provides
the concentrations of reactants and products at some point during the reaction. Based on this information, which of
the following explains whether or not the reaction has reached equilibrium, and why?
The reaction is not at equilibrium because ; the forward reaction is favored in order to form
(A)
more .
The reaction is not at equilibrium because ; the reverse reaction is favored in order to form
(B)
more and .
The reaction is not at equilibrium because ; the forward reaction is favored in order to form
(C)
more .
The reaction is at equilibrium because ; the amount of , and will remain the
(D)
constant.

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Unit 7 Progress Check: MCQ

Answer C

Correct. The expressions for and are similar: . When

, the reaction is not at equilibrium and will favor the forward reaction to form additional .

20.

Substance Initial Concentration ( )

0.10
0.10
0.10

The table lists the initial concentrations of each substance in the system represented by the equation above at a given
temperature. Which of the following best predicts what will occur as the system approaches equilibrium?
The rate of the reverse reaction will be less than the rate of the forward reaction, and additional
(A)
will be consumed because .
The rate of the reverse reaction will be greater than the rate of the forward reaction, and additional
(B)
will be consumed because .
The rate of the forward reaction will be less than the rate of the reverse reaction and, additional
(C)
will be produced because .
The rate of the forward reaction will be greater than the rate of the reverse reaction, and additional
(D)
will be produced because .

Answer B

Correct. , and when , the rate of the reverse reaction

will be greater than the rate of the forward reaction, resulting in the net consumption of
until equilibrium is reached.

21.

At a given temperature, the system represented by the chemical equation above is at equilibrium inside a rigid
container. Which of the following explains how the system will restore equilibrium, based on the correct
relationship between and , after a certain amount of is added?

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Unit 7 Progress Check: MCQ

Since , when is added to the system; the system will restore equilibrium
(A)
by producing more .

Since , when is added to the system; the system will restore equilibrium
(B)
by producing more .
Since , when is added to the system; the system will restore equilibrium
(C)
by producing more .
Since , when is added to the system; the system will restore equilibrium
(D)
by producing more .

Answer A

Correct. Based on the mathematical expression for , , the addition of increases the
numerator in this ratio, which increases the magnitude of overall. When , the system will
restore equilibrium by increasing the rate of the reverse reaction in the equilibrium and more
will be produced.

22. For which of the following salts would the relationship between molar solubility, , in , and the value of
be represented by the equation ?
(A)
(B)

(C)
(D)

Answer C

Correct. dissolves to form ions and ion. Letting represent the

molar solubility of , the expression for is
.

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Unit 7 Progress Check: MCQ

23.

A saturated solution of is at equilibrium at as represented by the equation above. If

at equilibrium, which of the following gives the correct molar solubility, , and for
?
(A)

(B)

(C)

(D)

Answer C

Correct. Based on the stoichiometry of the reaction, . Therefore,

.

24.

The dissolution of is represented above. Which of the following shows the mathematical relationship between
the molar solubility, , and the of at ?
(A)

(B)

(C)

(D)

Answer C

Correct. Since dissociates to form one mole of and one mole of ,

and the molar solubility of
.

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Unit 7 Progress Check: MCQ

25.

The equilibrium in a saturated solution of is represented above. In an experiment, a student places

of into of distilled water and stirs the mixture. How would the results be affected if the
student repeats the experiment but this time places of into of
instead of distilled water?
Less solid will dissolve, because the larger value of will cause the equilibrium position to lie
(A)
farther to the right.
Less solid will dissolve, because the larger value of will cause the equilibrium position to lie
(B)
farther to the left.
More solid will dissolve, because the larger value of will cause the equilibrium position to lie
(C)
farther to the right.
More solid will dissolve, because the smaller value of will cause the equilibrium position to lie
(D)
farther to the left.

Answer B

Correct. When starts to dissolve in the solution, the presence of the

ions already in the solution leads to more frequent collisions between and
, which increases the rate of the reverse (i.e., precipitation) reaction. An equilibrium becomes
established but with less of the dissolving than in the first experiment.

26.

The dissolution of solid is represented by the chemical equation above. In pure water, the equilibrium
concentration of ions in a saturated solution is . If a small amount of solid is added to
the saturated solution, which of the following would be observed?
(A) The increases and more dissolves.
(B) The increases and some precipitates.

(C) The molar solubility of becomes smaller than and some precipitates.
(D) The molar solubility of becomes larger than and more dissolves.

Answer C

Correct. Adding ions decreases the molar solubility of by increasing the rate at which the
reverse reaction in the dissolution equilibrium occurs, causing the precipitation of some .

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