The graph below represents the titration of 50 ml of

The graph below represents the titration of 50 mL of a 0.10 M acid with 0.10 M
NaOH
0
2
4
6
8
10
12
14
0 10 20 30 40 50 60
V base added (mL)
pH
Which of the following statements are consistent with this titration curve?
X. The equivalence point occurs when approximately 25 mL of NaOH are
added
Y. The curve represents the titration of a weak acid.
Z. The pK
a
of the acid is approximately 9.4.
A. X,Y, and Z
B. X,Y only
C. Y, Z only
D
. Y only
E. Z only
X. False. The equivalence point occurs when exactly 50 mL of NaOH are added.
Y. True. By the fact that the equivalence point is approximately 9 is consistent with a
strong base titrating a weak acid. In addition, by the fact that the initial point is
approximately 3 is consistent with the solution in the flask being a weak acid.
Z. False. Henderson-Hasselbalch equation: pH = pK
a
+ log([A
-
]/[HA])
[A
-
] = [HA] at the half-way point, which is 25 mL of titrant. At the half-way point
log([A
-
]/[HA]) = 0. Therefore, pH = pK
a
. Since at 25 mL of titrant (the half-way
point) the pH = 6.0, then pK
a
= 6.0.

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44 Chem 162-2007 Final exam + answers
Chapter 15B – Applications of Acid & Base Equilibria
Titrations and indicators
Calculate the pH after 10.0 mL of 0.400 M NaOH is added to 20.0 mL of 0.50 M
CH
3
COOH. (K
a
CH
3
COOH = 1.8 × 10
í 5
).
A. 2.52
B. 0.70
C. 13.70
D
. 4.57
E. 7.00
(0.400M NaOH x 0.0100L)/(0.0100L + 0.0200L) = 0.1333 M NaOH
(0.500M HA x 0.020L)/(0.020L + 0.010L) = 0.3333M HA
HA + OH
-
m
o
H
2
O + A
-
Reactions of strong bases go to completion.
HA +
OH
-
m
o
H
2
O + A
-
Initial 0.3333 0.1333 0
Change -0.1333 -0.1333 +0.1333
Equilibrium 0.2000 0 +0.1333
Henderson-Hasselbalch equation: pH = pK
a
+ log([A
-
]/[HA])
pH = -log(1.8 x 10
-5
) + log((0.1333)/(0.2000)) = 4.57
CHEM 162-2007 EXAM II + ANSWERS
CHAPTER 15B – APPLICATIONS OF ACID & BASE EQUILIBRIA (BUFFERS & TITRATIONS)
TITRATIONS AND INDICATORS
24. A titration curve is shown below. In this curve, the “titrant” refers to the substance in the buret,
which is added to another substance in the titrating flask.

39
This graph could represent:

40
SOLUBILITY PRODUCT
23 Chem 162-2011 Final exam