Question

Derive an expression for Ostwald’s dilution law.

Answer 1

Ostwald’s dflution law: 

It relates the dissociation constant of the weak acid (K₃) with its degree of dissociation (α) and the concentration (c). Considering a weak acid, acetic acid. The dissociation of acetic acid can be represented as, CH₃COOH ⇌ CH₃COO^- + H⁺ 

The dissociation constant of acetic acid is,

Substituting the equilibrium concentration in equation

We know that weak acid dissociates only to a very small extent compared to one, a is so small. equation (1) becomes,

Similarly, for a weak base,

The concentration of H can be calculated using the K value as below,

Substituting a value in equation (2),

For weak base

[OH^-] = \(\sqrt{K_b.C}\)