Calculate the pH of:
(i) an acidic buffer mixture 
(ii) a basic buffer mixture.
Or
Derive Henderson’s equation for an acidic and basic buffer mixture.
Or
Derive the following equation for the pH of an acidic buffer:

For acidic buffer mixture: Consider an acid buffer. Let it be a mixture of HA (weak acid) and its salt with a strong base, BA. A weak acid is only ionised to a small extent.

Applying the law of chemical equilibrium, we get,
                     
Due to the increase in the concentration of A^– ions the dissociation of the weak acid is further suppressed due to common ion effect. The concentration of weak acid molecules is taken as the concentration of HA and the concentration of the salt is the concentration of A^– ions. Thus,
[HA] = [Acid]
[BA] = [A^-] = [Salt]
Substituting the values of [HA] and [A^–] in expression (1), we have.

Taking logarithm of both the sides, we get,

This equation is known as Henderson equation. It helps in calculating the pH value of the buffer solution if the concentration of acid, as well as the salt, is known.
For basic buffer mixture: Proceeding exactly in a similar way as above, the expression for the pH of a basic buffer mixture (BOH + BA) can be obtained. BOH being a weak base dissociates to a small extent.

The salt BA being strong electrolyte dissociates almost completely. 
                             
Taking logarithm of both the sides we get,
     
This is also called Henderson equation. The pH of the basic buffer solution can be obtained by the following expression: