**Problem : **

What is the ratio of base to acid when pH =
p*K*_{a} in a buffer?
How about when pH = P*K*_{a} + 1?

pH = p*K*_{a} when the ratio of base to acid is 1 because log 1 =
0.
When log (base/acid)
= 1, then the ratio of base to acid is 10:1.

**Problem : **

Explain why the p*K*_{a} of a buffer should be as close as possible
to the desired pH.

The p*K*_{a} should be quite close to the desired pH so that the
ratio of base
to acid in the Henderson-Hasselbalch equation will be close to 1. As the
ratio of base to acid
deviates from 1, the addition of acids and bases to the buffer will have a
more profound effect on the
pH.

**Problem : **

What is the pH of a buffered solution of 0.5 M ammonia and 0.5 M ammonium
chloride when
enough hydrochloric acid is dissolved to make it 0.15 M HCl? The
p*K*_{b} of
ammonia is 4.75.

The p*K*_{a} of ammonium ion is 9.25 since p*K*_{a} =
14 - p*K*_{b}. 0.15 M
H^{+} reacts with
0.15 M ammonia to form 0.15 M more ammonium. Substituting the values of
0.65 M ammonium ion
(acid) and 0.35 M remaining ammonia (base) into the Henderson-Hasselbalch
equation
gives a pH of 8.98.