Acid-Base Equilibria And Buffer Solutions Worksheet With Answers Page 3
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5Chem 1B
Chapter 15 Exercises
7.
(a) Calculate the pH of the resulting solutions when (i) 2.0 mL of 6.0 M HCl
is added to pure water;
(aq)
(ii) 2.0 mL of 6.0 M NaOH is added pure water. In each case the final volume of the solution is 250 mL.
(Answer: (a) (i) pH = 1.32; (ii) pH = 12.68)
(b) How do changes in pH caused by the addition of a small amount of strong acid or strong base to a
buffer solution compare to that added to pure water?
(c) Explain the much smaller change in pH caused by the addition of strong acid or strong base
observed/calculated for the buffer solution. Illustrate your explanation with relevant equations.
8.
How many grams of sodium hydroxide must be added to a 500.-mL solution of 1.0 M acetic acid to
produce a buffer solution buffered with pH = 5.00? (Assume the final volume is 500. mL, and for acetic
–
5
acid, K
= 1.8 x 10
)
a
(Answer: 12.5 g of NaOH)
9.
(a) Explain buffering range and buffer capacity. (b) Can you change the buffering range of a given
buffer solution? (c) How would you increase the buffering capacity of a buffer solution?
10.
The K
values of some acids are given below:
a
–
–
4
5
Formic acid, HCOOH, K
= 1.8 x 10
; Acetic acid, CH
COOH, K
= 1.8 x 10
;
a
3
a
–
–
–
4
8
Lactic acid, HC
H
O
, K
= 1.4 x 10
; dihydrogenphosphate, H
PO
, K
= 6.2 x 10
;
3
5
3
a
2
4
a
-
–
–
5
11
Ammonia, NH
, K
= 1.8 x 10
, and hydrogen carbonate, HCO
, K
= 5.6 x 10
.
3
b
3
a
(a) Which acids and base solutions will you use to make buffers with the following pH?
(i) pH = 3.50;
(ii) pH = 5.00;
(iii) pH = 7.50
(iv) pH = 9.00
(v) pH = 10.50.
(b) Indicate the composition and the molar ratio of the conjugate base to the acid in each of the above
buffer solutions.
[
NaHCO
]
[
NaC
H
O
]
[
K
HPO
]
2
2
3
2
2
4
. 0
57
. 1
82
. 1
95
(Answers to (b): (i)
; (ii)
; (iii)
;
[
HCOOH
]
[
HC
H
O
]
[
KH
PO
]
2
3
2
2
4
[
NH
]
[
Na
CO
]
3
2
3
. 0
56
. 1
77
(iv)
; (v)
)
[
NH
Cl
]
[
NaHCO
]
4
3
3
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