Chemistry · Ch 3 — Ionic Equilibria

Dissociation constant of weak acids and weak bases

3.4.1

The dissociation of a weak acid HA in water is expressed as

\mathrm{HA(aq)} \rightleftharpoons \mathrm{H^{+}(aq)} + \mathrm{A^{-}(aq)}

The equilibrium constant, called the acid-dissociation constant, for this equilibrium is :

K_a = \frac{[\mathrm{H^{+}}][\mathrm{A^{-}}]}{[\mathrm{HA}]} \qquad \text{...(3.3)}

Similarly the dissociation of weak base BOH in water is represented as :

\mathrm{BOH(aq)} \rightleftharpoons \mathrm{B^{+}(aq)} + \mathrm{OH^{-}(aq)}

The equilibrium constant called the base-dissociation constant for this equilibrium is,

$K_b = \frac{[\mathrm{B^{+}}][\mathrm{OH^{-}}]}{[\mathrm{BOH}]} \qquad \text{...(3.4)}$ …