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
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