Weak acid is related is related to its conjugate base through equilibrium constant expression ka
HA +H2O ↔ H3O+ +A-
Ka =
Weak base is related is related to its conjugate acid through equilibrium constant expression kb
BOH + H2O ↔ B+ + OH-
Kb =
As the conjugated base pairs are related through the equilibrium constant we can assume that the equilibrium constant for an acid-base pair also related to each other.
Consider the dissociation of weak acid in water
HA +H2O ↔ H3O+ +A-
Ka = ...........(1)
HA being a weak acid it has strong conjugate base A-
The dissociation equilibrium of strong conjugate base in water is
A- + H2O ↔ HA +OH-
Kb = ...........(2)
Multiplying equation 1 & 2
Ka * Kb = *
= [H3O+ ][ OH-] = k w
k w = Ionic product of water
∴ ka * kb = k w
Common ion effect: The common ion effect is defined as the shift in the position of an ionic equilibrium caused by the addition of a solute that provides an ion which is the part of equilibrium.
Consider the dissociation of acetic acid in water
The equilibrium is represented as
CH3COOH (aq) ↔ CH3COO- +H3O+
To this solution add a small amount of sodium acetate.
dissociation of sodium acetate is represented as
CH3COONa → CH3COO- +Na+
Since the concentration of CH3COO- ions increased in the solution ,according to Le Chatelier,s principle the equilibrium will be shifted to left.
Due to the presence of common ions CH3COO- the equilibrium will be shifted to the left side. And the dissociation of CH3COOH is suppressed.