What is the pH?

Question

What is the pH of a buffer solution that is 0.120 M in hypochlorous acid (HClO) and 0.154 M in sodium hypochlorite? The K_a of hypochlorous acid is 2.9 × 10^-8.

Answer 1

Use the Henderson Hasselbalch equation:

pH = pKa + log [salt] / [acid]

pKa = -log Ka = -log 2.9x10-8 = 7.54

pH = 7.54 - log (0.154 / 0.120) = 7.54 - log 1.28 = 7.54 - 0.108

pH = 7.43