Name
Ch 1b, Problem Set Six SOLUTIONS
Section
Side 1 of 20
Due Friday, Feb. 19, 2010
Rec TA
at 4 PM PST in the Drop Box
1. Buffer Solutions (21 points)
An aqueous buffer is prepared by dissolving 0.13 mol of sodium sulfanilate
(NaNH
2
C
6
H
4
SO
3
)into1.00 L of 0.20 M sulfanilic acid (NH
2
C
6
H
4
SO
3
H; K
a
=5.9x10
4
) at
25
°
C.
(a)
Calculate the pH of the buffer solution at equilibrium. (
10 points
)
(b)
Suppose 0.040 mol of HCl is added to the buffer. Calculate the pH of the resulting
solution.
(
5 points
)
(c)
Briefly describe two changes you could make to improve the capacity of the buffer
solution described in the problem statement, without changing the chemicals used to
make the buffer.
(Note: Buffer capacity is defined as the number of moles of strong
acid, or strong base, needed to change the pH of one liter of buffer solution by 1 pH
unit.) (
4 points
)
(d)
A good rule of thumb is that “The useful pH range of a buffer is usually considered to
be pK
a
±
1 pH unit.”
Using this statement, determine the range through which the
ratio of the concentration of the acid to the concentration of the conjugate base is
most effective. (
2 points
)
Solution 1a
The acidbase equilibrium is:
NH
2
C
6
H
4
SO
3
H (
aq
) + H
2
O (
)
⁶
(NH
2
C
6
H
4
SO
3
)

(
aq
) + H
3
O
+
(
aq
).
Generate a table of initial and final values:
NH
2
C
6
H
4
SO
3
H
H
2
O
(NH
2
C
6
H
4
SO
3
)

H
3
O
+
Initial Conc. (M)
0.20
0.13
≈
0
Change
x
+x
+x
Equilib.Conc. (M)
0.20x
0.13+x
x
Note: Since the initial concentrations of the acid and its conjugate base are so large, we
can ignore the concentration of hydronium ions caused by the autoionization of water.
This approximation is usually valid for buffer solutions, but this approximation is not
always valid when adding a small amount of a weak base to water.
Use the definition of acid ionization constant,
, (
3 points
)
and substitute the equilibrium concentrations,
. (
3 points
)
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
Name
Ch 1b, Problem Set Six SOLUTIONS
Section
Side 2 of 20
Due Friday, Feb. 19, 2010
Rec TA
at 4 PM PST in the Drop Box
For buffer solutions, we can usually make the approximation that the change in
concentration (
x
) is small compared to the initial concentrations.
This is a very common
and useful strategy for doing buffer calculations. We’ll make this assumption and then
later check the validity of that assumption. Since x is very small,
.
.
Using the definition of pH,
, (
1 point
) (OGN5 p. 323; OGC6 p. 631)
and substituting the equilibrium concentration of the hydronium ion (
x
),
,
[1]
,
.
(
3 points total
: 2 points value; 1 point sig figs)
Note that
x
≈
1x10
3
, which is much smaller than 0.13; so our approximation is valid. (In
order for the approximation to be valid, x < 0.95 [HA])
GRADERS:
If students do not make the approximation that x<<0.13, then there should
be no penalty.
The answer will be pH=3.05 if no approximation is made.
NOTE TO STUDENTS
: If you didn’t make this approximation, you might consider it for
quizzes, exams and reallife – especially if you don’t solve quadratic equations quickly.
This is the end of the preview.
Sign up
to
access the rest of the document.
 Winter '11
 Reisman;Heath
 Chemistry, pH, graders, HClO, Section Rec TA

Click to edit the document details