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Special Topic I
119
Copyright © 2017 Pearson Education, Inc.
There are three ways to make such a buffer solution:
1. By mixing the appropriate amounts of acetic acid and sodium acetate in water and adding water
to obtain a final volume of 100 mL.
The amount of acetic acid needed: CH COOH 0.77 M
mmol
100 mL
0.77 M
77 mmol
3
[
]
=
=
=
=
=
M
mmol
mL
x
x
Therefore, we need 77 mmol of acetic acid in the final solution.
To obtain 77 mmol of acetic acid from a 1.50 M solution of acetic acid:
77
1 50
51 3
mmol
mL
M
mL
y
y
=
=
.
.
Notice that the formula
M
=
mmol
>
mL
is used twice. The first time it is used to determine the
number of mmol of acetic acid that is needed in the final solution. The second time it is used to
determine how that number of mmol can be obtained from an acetic acid solution with a known
concentration.
The amount of sodium acetate needed: [
]
CH COO 0.23 M
mmol
100 mL
0.23
23 mmol
3
−
=
=
=
x
x
To obtain 23 mmol of sodium acetate from a 1.50 M solution of sodium acetate:
23
1 50
mmol
mL
M
= 15.3 mL
y
y
=
.
The desired buffer solution can be prepared using: 51.3 mL 1.50 M acetic acid
15.3 mL 1.50 M sodium acetate
33.4 mL H
2
O
100.0 mL