Acid/base dissociation constant

An acid/base dissociation constant is a measure of the strength of an aqueous acid/base.

A weak acid dissociates partially in an aqueous solution to give the hydroxonium ion and a conjugate base.

$HA(aq)+H_2O(l)\rightleftharpoons H_3O^+(aq)+A^-(aq)$

Since water is the solvent, its activity approximately equals to one, and the equilibrium constant is given by:

$K_a=\frac{\left [ H_3O^+ \right ][A^-]}{[HA]}\; \; \; \; \; \; \; \; 12$

K_a is called the acid dissociation constant. The higher the value of K_a of an acid, the greater the extent of dissociation of the acid (i.e., the stronger the acid). It is difficult to determine accurately the K_a of an acid that completely ionises in water (a strong acid), since [HA] → 0. Hence, K_a is a useful measure only for weak acids.

A weak base dissociates partially in an aqueous solution to give a conjugate acid and the hydroxide ion.

$B(aq)+H_2O(l)\rightleftharpoons BH^+(aq)+OH^-(aq)$

Since water is the solvent, its activity approximately equals to one, and the equilibrium constant is given by:

$K_b=\frac{[BH^+][OH^-]}{[B]}\; \; \; \; \; \; \; \; 13$

K_b is called the base dissociation constant. The higher the value of K_b of a base, the greater the extent of dissociation of the base (i.e., the stronger the base). Similar to acids, it is difficult to determine accurately the K_b of a base that completely ionises in water (a strong base), since [B] → 0. Hence, K_b is a useful measure only for weak bases.