2006 AP® CHEMISTRY FREE-RESPONSE QUESTIONS
CHEMISTRY
Section II
(Total time—90 minutes)
Part A
Time— 40 minutes
YOU MAY USE YOUR CALCULATOR FOR PART A.
CLEARLY SHOW THE METHOD USED AND THE STEPS INVOLVED IN ARRIVING AT YOUR ANSWERS.
It is to your advantage to do this, since you may obtain partial credit if you do and you will receive little or no credit if
you do not. Attention should be paid to significant figures.
Be sure to write all your answers to the questions on the lined pages following each question in the booklet with the
pink cover. Do NOT write your answers on the green insert.
Answer Question 1 below. The Section II score weighting for this question is 20 percent.
1. Answer the following questions that relate to solubility of salts of lead and barium.
(a) A saturated solution is prepared by adding excess PbI2(s) to distilled water to form 1.0 L of solution at
25°C. The concentration of Pb2+ (aq) in the saturated solution is found to be 1.3 × 10 − 3 M . The chemical
equation for the dissolution of PbI2(s) in water is shown below.
→ Pb2+(aq) + 2 I −(aq)
PbI2(s) ←
(i) Write the equilibrium-constant expression for the equation.
(ii) Calculate the molar concentration of I −(aq) in the solution.
(iii) Calculate the value of the equilibrium constant, Ksp .
(b) A saturated solution is prepared by adding PbI2(s) to distilled water to form 2.0 L of solution at 25°C. What
are the molar concentrations of Pb2+ (aq) and I −(aq) in the solution? Justify your answer.
(c) Solid NaI is added to a saturated solution of PbI2 at 25°C. Assuming that the volume of the solution does
not change, does the molar concentration of Pb2+ (aq) in the solution increase, decrease, or remain the
same? Justify your answer.
(d) The value of Ksp for the salt BaCrO4 is 1.2 × 10−10. When a 500. mL sample of 8.2 × 10− 6 M Ba(NO3)2
is added to 500. mL of 8.2 × 10− 6 M Na2CrO4 , no precipitate is observed.
(i) Assuming that volumes are additive, calculate the molar concentrations of Ba2+ (aq) and CrO42−(aq)
in the 1.00 L of solution.
(ii) Use the molar concentrations of Ba2+ (aq) ions and CrO42−(aq) ions as determined above to show
why a precipitate does not form. You must include a calculation as part of your answer.
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AP® CHEMISTRY
2006 SCORING GUIDELINES
Question 1
1. Answer the following questions that relate to solubility of salts of lead and barium.
(a) A saturated solution is prepared by adding excess PbI2(s) to distilled water to form 1.0 L of solution
at 25°C. The concentration of Pb2+ (aq) in the saturated solution is found to be 1.3 × 10 − 3 M . The
chemical equation for the dissolution of PbI2(s) in water is shown below.
→ Pb2+(aq) + 2 I −(aq)
PbI2(s) ←
(i) Write the equilibrium-constant expression for the equation.
K sp = [Pb 2+ ][I− ]2
One point is earned for the correct expression.
(ii) Calculate the molar concentration of I −(aq) in the solution.
By stoichiometry, [I− ] = 2 × [Pb2+] ,
thus [I− ] = 2 × (1.3 × 10 − 3 ) = 2.6 × 10 − 3 M
One point is earned for the correct concentration.
(iii) Calculate the value of the equilibrium constant, Ksp .
Ksp = [Pb2+ ][I−]2 = (1.3 × 10−3)(2.6 × 10−3) 2
One point is earned for a value of Ksp that is
consistent with the answers in parts (a)(i) and (a)(ii).
= 8.8 × 10−9
(b) A saturated solution is prepared by adding PbI2(s) to distilled water to form 2.0 L of solution at 25°C.
What are the molar concentrations of Pb2+ (aq) and I −(aq) in the solution? Justify your answer.
The molar concentrations of Pb2+ (aq) and I −(aq)
would be the same as in the 1.0 L solution in part (a)
(i.e., 1.3 × 10 − 3 M and 2.6 × 10 − 3 M , respectively).
The concentrations of solute particles in a saturated
solution are a function of the constant, Ksp , which is
independent of volume.
One point is earned for the concentrations
(or stating they are the same as in the solution
described in part (a)) and justification.
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AP® CHEMISTRY
2006 SCORING GUIDELINES
Question 1 (continued)
(c) Solid NaI is added to a saturated solution of PbI2 at 25°C. Assuming that the volume of the solution
does not change, does the molar concentration of Pb2+ (aq) in the solution increase, decrease, or remain
the same? Justify your answer.
[Pb2+] will decrease.
The NaI(s) will dissolve, increasing [I− ] ; more I−(aq) then combines
with Pb2+(aq) to precipitate PbI2(s) so that the ion product [Pb2+][I− ] 2
will once again attain the value of 8.8 × 10− 9 (Ksp at 25°C).
One point is earned for
stating that [Pb2+] will
decrease.
One point is earned for
justification (can involve a
Le Chatelier argument).
(d) The value of Ksp for the salt BaCrO4 is 1.2 × 10−10. When a 500. mL sample of 8.2 × 10− 6 M
Ba(NO3)2 is added to 500. mL of 8.2 × 10− 6 M Na2CrO4 , no precipitate is observed.
(i) Assuming that volumes are additive, calculate the molar concentrations of Ba2+ (aq) and
CrO42−(aq) in the 1.00 L of solution.
New volume = 500. mL + 500. mL = 1.000 L , therefore [Ba2+]
in 1.000 L is one-half its initial value:
[Ba2+] =
500. mL
× (8.2 × 10− 6 M ) = 4.1 × 10− 6 M
1,000. mL
[CrO42−] =
One point is earned for the
correct concentration.
500. mL
× (8.2 × 10− 6 M ) = 4.1 × 10− 6 M
1,000. mL
(ii) Use the molar concentrations of Ba2+ (aq) ions and CrO42−(aq) ions as determined above to show
why a precipitate does not form. You must include a calculation as part of your answer.
The product Q = [Ba2+][CrO42−]
One point is earned for calculating a value of
Q that is consistent with the concentration
values in part (d)(i).
= (4.1 × 10− 6 M)(4.1 × 10− 6 M)
= 1.7 × 10− 11
Because Q = 1.7 × 10− 11 < 1.2 × 10−10 = Ksp ,
no precipitate forms.
One point is earned for
using Q to explain why no precipitate forms.
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AP® CHEMISTRY
2006 SCORING COMMENTARY
Question 1
Overview
This was a required equilibrium problem designed to assess students’ understanding of solubility and principles
of solubility equilibrium. Students were expected to write an expression for the Ksp from the solubility equation,
determine the stoichiometric relationship between concentrations of ions, and calculate the value of the Ksp .
They were asked to assess the effect of different volumes of solution on the concentrations of ions in a saturated
solution and the effect of adding a soluble salt containing a common ion. The last part of the question assessed
students’ ability to calculate concentrations of ions from dilution data and to calculate and interpret the value of
Q in comparison to Ksp .
Sample: 1A
Score: 9
This response earned all 9 points: 1 point for part (a)(i), 1 point for part (a)(ii), 1 point for part (a)(iii), 1 point for
part (b), 2 points for part (c), 1 point for part (d)(i), and 2 points for part (d)(ii).
Sample: 1B
Score: 7
All of the points were earned in parts (a), (b), and (d). The points were not earned in part (c) because the student
does not show that an increase in [I − ] results in a decrease in [Pb2+].
Sample: 1C
Score: 5
The point was not earned in part (a)(i) because solid PbI2 is included in the expression. Part (a)(iii) is not
attempted. The point was not earned in part (b) because the concentrations are halved. In part (c) 1 point was
earned for stating that [Pb2+] decreases; however, the justification is insufficient to have earned the second point.
All 3 points were earned in part (d).
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Student Performance Q&A:
2006 AP® Chemistry Free-Response Questions
The following comments on the 2006 free-response questions for AP® Chemistry were written by
the Chief Reader, Eleanor Siebert of Mount St. Mary’s College in Los Angeles, California. They
give an overview of each free-response question and of how students performed on the question,
including typical student errors. General comments regarding the skills and content that students
frequently have the most problems with are included. Some suggestions for improving student
performance in these areas are also provided. Teachers are encouraged to attend a College Board
workshop to learn strategies for improving student performance in specific areas.
Question 1
What was the intent of this question?
This question was a required equilibrium problem designed to assess students’ understanding of solubility
and principles of solubility equilibrium. Students were expected to write an expression for the Ksp from
the solubility equation, determine the stoichiometric relationship between concentrations of ions, and
calculate the value of the Ksp . They were asked to assess the effect of different volumes of solution on the
concentrations of ions in a saturated solution and the effect of adding a soluble salt containing a common
ion. The last part of the question assessed students’ ability to calculate concentrations of ions from
dilution data and to calculate and interpret the value of Q in comparison to Ksp .
How well did students perform on this question?
Students did a good job answering the question, earning a mean score of 4.19 out of a possible 9 points.
There was a very flat score distribution, with each score (0–9) accounting for 8 to 12 percent of the
responses (excluding blanks). The comparatively large percentage of 8s and 9s indicated that many
students had mastered the tested concepts. The points were well distributed among the four parts of the
problem, though many responses that scored in the midrange (5–6) earned most of the points on parts (a)
and (d). Students who earned 1–3 points usually earned them in part (a). Part (b) was the most challenging
for students; most of the students who earned 8 points failed to earn the 1 point for part (b).
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What were common student errors or omissions?
Part (a)(i): Some students included [PbI2] in the equilibrium constant expression, and some omitted the
charges on Pb2+ and I−.
Part (a)(ii): Some students indicated that [I−] = (1.3 × 10−3) 2 = 1.7 × 10− 6, and some made errors with
significant figures.
Part (a)(ii) and (a)(iii): Some students made errors with significant figures.
Part (b): Many students indicated that the concentration was inversely (or directly) proportional to
volume. Many stated (correctly) that concentration is independent of volume but failed to refer to the
saturated solution, equilibrium, or Ksp .
Part (c): Many students stated that solid NaI would have no effect on the equilibrium constant. Incorrect
explanations included indicating that the change in volume causes a dilution that affects [Pb2+], that the
added Na+ crowds the Pb2+, and that if [I−] increases [Pb2+] also has to increase.
Part (d)(i): Some students failed to use the concentration and volume data given and calculated [Ba2+] and
[CrO42−] (incorrectly) from the given Ksp .
Part (d)(ii): Many students indicated that since Q ≠ Ksp , no precipitate forms, while others indicated that
if [Ba2+] = [CrO42−], no precipitate forms.
Based on your experience of student responses at the AP Reading, what message would
you like to send to teachers that might help them to improve the performance of their
students on the exam?
•
•
•
•
Stress the distinction between amount (moles), volume (liters), and concentration (molarity), and
which (if any) of these quantities remains unchanged in various lab situations (dilution,
equilibrium, etc.).
Stress the interpretation of numerical data and “reasonable” values. Students should be able to
estimate order-of-magnitude values for quantities without performing calculations and should be
able to recognize chemically absurd values.
Differentiate between experimental contexts that are described by different types of equilibrium
constants: Keq , Kp , Kc , Ksp , Ka , Kb .
Students should understand reaction stoichiometry and be able to use that to calculate the
concentration of molecular and ionic species in solution.
Question 2
What was the intent of this question?
The intent of this question was to test students’ knowledge of basic thermodynamic relationships,
including enthalpy, entropy, and free energy changes, and the equilibrium constant associated with a
chemical reaction. The first task was to calculate the values for enthalpy, entropy, and free energy
changes from the information provided. Students were asked what the calculated thermodynamic
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