We are asked to calculate the pH of the buffer after 0.012 mol of NaOH is added.
The buffer solution is composed of HA and A-.
HA is a weak acid and based on the Bronsted-Lowry definition, an acid is a proton (H+). Once A- loses its proton, its conjugate base is formed:
HA(aq) + H2O(l) → A-(aq) + H3O+(aq)
(weak acid) (base) (conjugate base) (conjugate acid)
Whenever we have a conjugate base and a weak acid, we have a buffer.
A buffer that contains 0.13 M of an acid, HA and 0.33 M of its conjugate base A-, has a pH of 4.27. What is the pH after 0.012 mol of NaOH are added to 0.73 L of the solution?