What is the expected pH of a 0.723M solution of hypochlorous acid in water?

Question

What is the expected pH of a 0.724M solution of hypochlorous acid in water? Ka(HClO) = 2.931x10^-8

Answer 1

HClO <==> H⁺ + ClO^-

Ka = [H⁺][ClO^-] / [HClO]

2.93x10^-8 = (x)(x) / 0.723 - x and if we assume x is small relative to 0.723, we can ignore it in denominator

2.93x10^-8 = (x)(x) / 0.723

x² = 2.12x10^-8

x = [H^+] = 1.46x10^-4 (this is very small relative to 0.723 so above assumption was valid)

pH = -log [H+]

pH = -log 1.46x10^-4

pH = 3.84