Question on pH of a solution

What will be the pH of the solution obtained by mixing equal volumes of 0.1M HOAc with 0.1M NaOH if pKa of HOAc = 4.74?

Answer:

Equal volumes of 0.1M NaOH and 0.1M HOAc react to give 0.05M NaOAc. An aqueous solution of NaOAc will be basic as it is a salt of a weak acid and a strong base.

The pH of an aqueous solution of a weak acid and a strong base is calculated as follows;

pH = ½ pK_w + ½ pK_a + ½ log C = ½ (14) + ½ (4.74) + ½ log (0.05) = 8.72 

(pK_w = 14 at 25°C)