Problem :
What is the ratio of base to acid when pH =
pK_{a} in a buffer?
How about when pH = PK_{a} + 1?
pH = pK_{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 pK_{a} of a buffer should be as close as possible
to the desired pH.
The pK_{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
pK_{b} of
ammonia is 4.75.
The pK_{a} of ammonium ion is 9.25 since pK_{a} =
14 - pK_{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.