pH of 0.08 mol dm-3 HOCl

Question:
pH of 0.08 mol dm-3 HOCl solution is 2.85. Calculate its ionisation constant.

Solution:

Given, pH =2.85 and C =0.08 mol dm-3

Now since HOCl is a weak acid its dissociation will be given as-

HOCl + H2O ⇌ H30+ + OCl-

We know that;

pH = -log [H+]

-2.85 = log [H+] [H+] = antilog (-2.85)

[H+] = 1.41 × 10-3

We also know that, for a weak mono basic acid-

(On squaring both sides) [H+]2 = kaC

Ka = (1.41 x 10-3)2/ 0.08 = 2.5 × 10-5

Hence the ionization constant of HOCl will be 2.5 × 10-5