AP CHEMISTRY
Test Booklet
8.8 Properties of buffers
1.
A solution is prepared by mixing equal volumes of
following correctly describes what occurs if a small amount of
and
or
. Which of the
is added?
(A)
If
ions.
is added, the
will increase only slightly because the
ions will react with
(B)
If
ions.
is added, the
will decrease only slightly because the
ions will react with
(C)
(D)
If
is added, the
ions.
will increase only slightly because the
ions will react with
If
is added, the
molecules.
will decrease only slightly because the
ions will react with
2.
A buffer solution is made up of acetic acid (
) and sodium acetate (
). The major
equilibria in the buffer system are represented above. Which of the following equilibria could be used to support the
claim that the addition of a small amount of
to the buffer will result in only a very small change in
?
(A)
(B)
(C)
(D)
3.
A solution is prepared by adding 100 mL of 1.0 M HC2H3O2(aq) to 100 mL of 1.0 M NaC2H3O2(aq). The solution
is stirred and its pH is measured to be 4.73. After 3 drops of 1.0 M HCl are added to the solution, the pH of the
solution is measured and is still 4.73. Which of the following equations represents the chemical reaction that
accounts for the fact that acid was added but there was no detectable change in pH?
(A) H3O+(aq) + OH-(aq) → 2 H2O(l)
(B) H3O+(aq) + Cl-(aq) → HCl(g) + H2O(l)
(C)
H3O+(aq) + C2H3O2-(aq) → HC2H3O2(aq) +H2O(l)
(D) H3O+(aq) + HC2H3O2(aq) → H2C2H3O2+(aq) + H2O(l)
4.
A student prepares a solution by combining 100 mL of 0.30 M HNO2(aq) and 100 mL of 0.30 M KNO2(aq). Which
of the following equations represents the reaction that best helps to explain why adding a few drops of 1.0 M
HCl(aq) does not significantly change the pH of the solution?
(A) K+(aq) + Cl−(aq) → KCl(s)
(B)
HNO2(aq) → H+(aq) + NO2−(aq)
(C)
H+(aq) + OH−(aq) → H2O(l)
(D) H+(aq) + NO2−(aq) → HNO2(aq)
AP Chemistry
Page 1 of 10
Test Booklet
8.8 Properties of buffers
5.
When a small amount of 12 M HNO3(aq) is added to a buffer solution made by mixing CH3 NH2(aq) and
CH3NH3Cl(aq), the pH of the buffer solution changes from 10.64 to 10.62. Which of the following equations
represents the reaction that accounts for the fact that the pH does not change significantly when the HNO3(aq) is
added?
(A) CH3NH2(aq) + H+(aq) → CH3NH3+(aq)
(B) CH3NH3(aq) + H+(aq) → CH3NH42+(aq)
(C)
NO3-(aq) + H+(aq) → HNO3(aq)
(D) OH-(aq) + H+(aq) → H2O(l)
6.
Mixtures that would be considered buffers include which of the following?
I. 0.10 M HCl + 0.10 M NaCl
II. 0.10 M HF + 0.10 M NaF
III. 0.10 M HBr + 0.10 M NaBr
(A) I only
(B)
II only
(C)
III only
(D) I and II
(E)
Page 2 of 10
II and III
AP Chemistry
Test Booklet
8.8 Properties of buffers
7.
For parts of the free response question that require calculations, clearly show the method used and the steps
involved in arriving at your answers. You must show your work to receive credit for your answer. Examples and
equations may be included in your answers where appropriate.
Methylamine is a weak base with the formula
.
(a) In the following box, complete the Lewis electron-dot diagram for a molecule of methylamine. Show all
bonding and nonbonding valence electrons.
Please respond on separate paper, following directions from your teacher.
The following chemical equation represents the reaction that occurs when methylamine dissolves in water to form
a basic solution.
(b) The
of
is 12.54. Determine the value of
for methylamine.
Please respond on separate paper, following directions from your teacher.
AP Chemistry
Page 3 of 10
Test Booklet
8.8 Properties of buffers
A
sample of a
Following is a graph that shows
(c) If
the
of
solution of unknown concentration is titrated with
versus the volume of
added during the titration.
.
was required to reach the equivalence point, calculate the concentration of
solution of unknown concentration.
Please respond on separate paper, following directions from your teacher.
(d) Using the symbols in the legend above, draw particles in the following beaker to represent the relative amounts
of the two species,
and
, in the solution after the first
of titrant had been added to
the
solution.
Please respond on separate paper, following directions from your teacher.
Page 4 of 10
AP Chemistry
Test Booklet
8.8 Properties of buffers
(e) Explain why the titration curve shows only a small change in
per volume of acid added when the total
amount of acid added is about
. Include a balanced chemical equation as part of your answer.
Please respond on separate paper, following directions from your teacher.
8.
The acid ionization equilibrium for
is represented by the chemical equation above. A student claims that the
of a solution that contains
and
will change only slightly when small
amounts of acids or bases are added. Which of the following pairs of equations can the student use to justify the
claim?
and
(A)
(B)
and
(C)
and
(D)
and
AP Chemistry
Page 5 of 10
Test Booklet
8.8 Properties of buffers
Refer to the following.
The pH of solutions of four acids prepared at various concentrations were measured and recorded in the table above.
The four acids are, in no particular order, chlorous, hydrochloric, lactic, and propanoic.
9.
A 25 mL sample of a 1.0 M solution of acid 1 is mixed with 25 mL of 0.50 M NaOH. Which of the following best
explains what happens to the pH of the mixture when a few drops of 1.0 M HNO3 are added?
(A) The pH of the mixture increases sharply, because HNO3 is a strong acid.
(B)
The pH of the mixture decreases sharply, because H3O+ ions were added.
(C)
The pH of the mixture stays about the same, because the conjugate base of acid 1 reacts with the added H3O+
ions.
(D)
The pH of the mixture stays about the same, because the OH− ions in the solution react with the added H3O+
ions.
The equilibrium for the reaction between
table shows the
of three solutions of
10.
0.050
11.69
0.10
11.85
0.20
12.01
, a weak base, and water is represented by the equation below. The
at
.
of
with
of
and
A student mixes
claims that if a small amount of strong base is added to the mixture, then the resulting change in
of the mixture
that would result from adding the same amount of strong base to
of
will be smaller than the change in
. Which of the following best explains whether or not the student’s claim is correct?
Page 6 of 10
AP Chemistry
Test Booklet
8.8 Properties of buffers
The claim is correct because the mixture contains only half of the
that the
contains. The lesser amount of base in the mixture makes the
of the mixture
of the
; therefore, when a small amount of strong base is
(A) lower than the
of the mixture will be smaller than the change in the
of the
added, the change in the
.
(B)
,
The claim is correct because the mixture contains a significant amount of the acid
. In
which can react with and partially neutralize the added strong base, thereby reducing the change in
, the concentration of the acid
is very small; therefore, the
will be larger.
added strong base is not neutralized and the change in
(C)
The claim is incorrect because the mixture contains only half of the
that the
contains. The lesser amount of base in the mixture makes the
of the mixture
of the
; therefore, when a small amount of strong base is
lower than the
of the mixture will be larger than the change in the
of the
added, the change in the
.
The claim is incorrect because the change in
resulting from the addition of a small amount of strong base
to
the
mixture
and
to
the
will be the same given that the same amount of strong
(D)
base is being added to the same volume of solution.
AP Chemistry
Page 7 of 10
Test Booklet
8.8 Properties of buffers
11.
A student prepares three solutions, X, Y, and Z, as described in the table above. The values of Ka for the acidic
species in the solutions are given in the table below.
a. Using the information above, write the letters of the solutions in the boxes below to rank the solutions in
order of increasing pH. Explain your reasoning for the ranking.
b. Does the pH of solution Y increase, decrease, or remain the same when 100 mL of water is added?
Justify your answer.
c. The student adds 0.0010 mol of NaOH(s) to solution Y, and adds 0.0010 mol of NaOH(s) to solution Z.
Assume that the volume of each solution does not change when the NaOH(s) is added. The pH of
solution Y changes much more than the pH of solution Z changes. Explain this observation.
Please respond on separate paper, following directions from your teacher.
Page 8 of 10
AP Chemistry
Test Booklet
8.8 Properties of buffers
12.
For parts of the free-response question that require calculations, clearly show the method used and the
steps involved in arriving at your answers. You must show your work to receive credit for your answer.
Examples and equations may be included in your answers where appropriate.
The reaction between hypochlorous acid and water is represented above.
(a) Identify one of the conjugate acid-base pairs in the reaction.
Please respond on separate paper, following directions from your teacher.
(b) Shown in the graph below is the titration curve that results when
of
is titrated
with
. Carefully draw a second curve on the graph that would result from the titration of
of
with
.
Please respond on separate paper, following directions from your teacher.
of
(c) A student proposes creating a buffer by dissolving
. Explain why the resulting solution would not be a buffer.
in
AP Chemistry
of
Page 9 of 10
Test Booklet
8.8 Properties of buffers
Please respond on separate paper, following directions from your teacher.
The graph below shows the titration curve that results when 100. mL of 0.0250 M acetic acid is titrated with 0.100 M
NaOH.
13.
What part of the curve corresponds to the optimum buffer action for the acetic acid/acetate ion pair?
(A) Point V
(B)
Point X
(C)
Point Z
(D) Along all of section WY
(E)
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Along all of section YZ
AP Chemistry