Question

HClO is a weak acid (Ka = 4.0 × 10–8) and so the salt NaClO acts as a weak base. What is the pH of a solution that is 0.055 M in NaClO at 25 °C?

Answer 1

first find the Kb value from Ka

Ka x Kb = 1.0 x 10^-14

Kb = 1.0 x 10^-14 / 4.0 x 10^-8 = 2.5 x 10^-7

construct ICE table

NaClO (aq) + H2O (l) <----> HClO (aq) + NaOH (aq)

I 0.055 0 0

C -x +x +x

E 0.055 -x +x +x

Kb = [HClO] [NaOH] / [NaOCl]

2.5 x 10^-7 = [x] [x] / [0.055-x]

x^2 + x 2.5 * 10^-7 - 1.375 * 10^-8 = 0

solve the quadratic equation

x = 0.0001171 = [NaOH]

pOH = -loh[OH-] = -log[0.0001171] = 3.93

pH + pOH = 14

pH = 14-pOH = 14 - 3.93 = 10.06

pH = 10.06