Common ion effect

Common ion effect is the change in the ionization equilibrium due to the addition of a solute that provides an ion taking part in the equilibrium. Let us take an example with acetic acid.

Example 3.1 : You are given a 0.1 M solution of acetic acid.

•  Calculate the concentrations of all the chemical species present at equilibrium (Given K_a of acetic acid is 1.7 × 10^-5M ).

•  Calculate the concentrations of all the species when 0.05 M of sodium acetate is added to the solutions.

(3.9) (3.10)

If the acid is <10% ionized i.e. if K_a < 10^-3, 0.1–x can be approximate to 0.1. Equation 3.10, therefore, reduces to:

(3.11) (3.12)

Therefore, the equilibrium concentrations of CH₃COOH, CH₃COO⁻, and H₃O⁺ are 98.7 mM , 1.3 mM , and 1.3 mM , respectively.

Solution (b) : Sodium acetate dissociates completely in aqueous solutions; 0.05 M sodium acetate therefore provides 0.05 M of acetate ions.

(3.13) (3.14)