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130 Special Topic I
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11.
Because formic acid has a
p
K
a
=
3.75,
a formic acid/formate buffer can be a buffer at
pH
=
3.30,
because
this pH is within one pH unit of the
p
K
a
value.
fraction of buffer in the basic form
=
+
=
×
×
+
−
K
K
a
a
4
H
1.78 10
(1 10
10
[ ]
78
×
+
−
−
4
4
.
) ( .5 01
= ×
×
=
−
−
1 10
6.79 10
0.26
4
4
.78
)
The solution must have
3
formic acid
4
=
0.74 M
and
3
sodium formate
4
=
0.26 M.
12.
The reaction to be carried out produces protons that will react with the basic form of the buffer in order to
keep the pH constant.
Therefore, the better buffer is the one that has the larger percentage of the buffer in the basic form.
The
p
K
a
of formic acid is 3.74. Because the pH of the solution (4.2) is greater than the
p
K
a
of the compound,
formic acid will be primarily in its basic form.
The
p
K
a
of acetic acid is 4.76. Because the pH of the solution (4.2) is less than the
p
K
a
of the compound,
acetic acid will be primarily in its acidic form.
Therefore, the formate buffer is preferred, because it has a greater percentage of the buffer in the basic form.