AP
®
CHEMISTRY
2012 SCORING GUIDELINES
© 2012 The College Board.
Visit the College Board on the Web: www.collegeboard.org.
Question 1
(10 points)
A 1.22 g sample of a pure monoprotic acid,
HA, was dissolved in distilled water. The HA solution was then
titrated with 0.250 M
NaOH. The pH was measured throughout the titration, and the equivalence point was
reached when 40.0 mL of the
NaOH solution had been added. The data from the titration are recorded in the
table below.
Volume of 0.250 M NaOH
Added (mL)
pH of Titrated
Solution
0.00
?
10.0
3.72
20.0
4.20
30.0
?
40.0
8.62
50.0
12.40
(a) Explain how the data in the table above provide evidence that
HA is a weak acid rather than a strong acid.
The
pH at the equivalence point is above 7, which
indicates that
HA is a weak acid.
1 point is earned for the correct explanation.
(b) Write the balanced net-ionic equation for the reaction that occurs when the solution of
NaOH is added to the
solution of
HA .
HA(aq) + OH
(aq) A
(aq) + H
2
O(l)
1 point is earned for writing the net-ionic
equation balanced for mass and charge.
(c) Calculate the number of moles of
HA that were titrated.
At the equivalence point, the number of moles of base added equals the
number of moles of acid initially present.
0.250 mol NaOH 1 mol HA
0.0400 L
L1 mol NaOH
= 0.0100 mol HA
1 point is earned for the correct
number of moles.
AP
®
CHEMISTRY
2012 SCORING GUIDELINES
© 2012 The College Board.
Visit the College Board on the Web: www.collegeboard.org.
Question 1 (continued)
(d) Calculate the molar mass of
HA .
massof acid 1.22 g
MM 122g/mol
moles of acid 0.0100mol
1 point is earned for the correct molar mass.
The equation for the dissociation reaction of HA in water is shown below.
HA(aq) + H
2
O(l)
H
3
O
+
(aq) + A
(aq) K
a
= 6.3 × 10
5
(e) Assume that the initial concentration of the
HA solution (before any NaOH solution was added) is 0.200 M.
Determine the pH of the initial
HA solution.
3
5
3
3
3
3
[H O ][A ]
[HA]
()()
6.3 10 ; assume that << 0.200 .
(0.200 )
[H O ] 3.5 10
pH = log[H O ] log(3.5 10 ) 2.45
a
K
xx
x
M
x
xM
1 point is earned for the appropriate
substitution into the
K
a
expression.
1 point is earned for the correct
[H
3
O
+
].
1 point is earned for the calculation of
pH.
(f) Calculate the value of [H
3
O
+
] in the solution after 30.0 mL of NaOH solution is added and the total volume
of the solution is 80.0 mL.
HA + OH
A
+ H
2
O
mol before rxn: 0.0100 0.00750 0.00000
mol after rxn: 0.00250 0.00000 0.00750
2
2
3
2
5
2
22
5
0.00250mol
[HA] 3.13 10
0.0800L
0.00750mol
[A ] 9.38 10
0.0800 L
[H O ][A ]
[HA]
()(9.38 10 )
6.3 10
(3.13 10 )
Assume that << 9.38 10 and 3.13 10 ,
()(9.38 1
then 6.3 10
a
M
M
K
xx
x
x
MM
x
2
2
5
3
0)
(3.13 10 )
[H O ] 2.10 10 .xM
1 point is earned for the correct calculation
of moles of
A
and HA after the reaction.
1 point is earned for the appropriate
substitution into the equilibrium expression.
1 point is earned for the correct
calculation of
[H
3
O
+
].
© 2012 The College Board.
Visit the College Board on the Web: www.collegeboard.org.
© 2012 The College Board.
Visit the College Board on the Web: www.collegeboard.org.
© 2012 The College Board.
Visit the College Board on the Web: www.collegeboard.org.
© 2012 The College Board.
Visit the College Board on the Web: www.collegeboard.org.
© 2012 The College Board.
Visit the College Board on the Web: www.collegeboard.org.
© 2012 The College Board.
Visit the College Board on the Web: www.collegeboard.org.
AP
®
CHEMISTRY
2012 SCORING COMMENTARY
© 2012 The College Board.
Visit the College Board on the Web: www.collegeboard.org.
Question 1
Overview
This question assessed students’ understanding of and ability to solve problems and explain concepts that
pertain to a weak acid/strong base titration. Part (a) asked students to provide evidence that HA is a weak
acid. Part (b) required students to write a balanced net-ionic equation for the reaction between HA and
NaOH. Part (c) asked students to calculate the number of moles of HA titrated. In part (d) students were
asked to calculate the molar mass of HA. In part (e) students were given the ionization equation and the
K
a
value for the weak acid and were asked to determine the pH of HA before addition of any NaOH. Part (f)
assessed the students’ understanding of the titration process by asking students to calculate [H
3
O
+
] at a
point in the titration before the equivalence point.
Sample: 1A
Score: 10
The response earned all 10 available points.
Sample: 1B
Score: 8
In part (e) the response earned the first point for the appropriate substitution into the K
a
expression, but
did not earn the second point because the [H
3
O
+
] was calculated incorrectly. The third point was earned
for calculating a pH consistent with the incorrect [H
3
O
+
]. In part (f) the response earned the first point for
calculating the values of HA and A
−
and the second point for substituting the values into the Henderson-
Hasselbalch equation but did not earn the third point for the value of [H
3
O
+
].
Sample: 1C
Score: 6
In part (b) the response did not earn the point because the equation used is not the correct net ionic
equation. In part (f) the response earned no points because the student multiplies the concentration and
volume for the acid and base but does not complete the calculation.
