# JEE Main & Advanced Chemistry Equilibrium / साम्यावस्था Dissociation Constants Of Acids And Bases

Dissociation Constants Of Acids And Bases

Category : JEE Main & Advanced

(1) Dissociation constant for weak acid : Consider an acid $HA$ which, when dissolved in water ionizes as,

$HA$  $\rightleftharpoons$  ${{H}^{+}}+{{A}^{-}}$

Applying the law of mass action, ${{K}_{a}}=\frac{[{{H}^{+}}][{{A}^{-}}]}{[HA]}$

Where, ${{K}_{a}}$ is the dissociation constant of the acid, $HA$. It has constant value at definite temperature and does not change with the change of concentration.

Dissociation Constant for polybasic acid : Polybasic acids ionise stepwise as, for example, orthophosphoric acid ionises in three steps and each step has its own ionisation constant.

${{H}_{3}}P{{O}_{4}}$  $\rightleftharpoons$ ${{H}^{+}}+{{H}_{2}}PO_{4}^{-}$              (I step)

${{H}_{2}}PO_{4}^{-}$  $\rightleftharpoons$  ${{H}^{+}}+HPO_{4}^{-2}$                      (II step)

$HPO_{4}^{-2}$  $\rightleftharpoons$ ${{H}^{+}}+PO_{4}^{-3}$                                   (III step)

Let ${{K}_{1}},\ {{K}_{2}}$ and ${{K}_{3}}$ be the ionization constants of first, second and third steps respectively. Thus,

${{K}_{1}}=\frac{[{{H}^{+}}][{{H}_{2}}PO_{4}^{-}]}{[{{H}_{3}}P{{O}_{4}}]}$;${{K}_{2}}=\frac{[{{H}^{+}}][HPO_{4}^{-2}]}{[{{H}_{2}}PO_{4}^{-}]}$;${{K}_{3}}=\frac{[{{H}^{+}}][PO_{4}^{-3}]}{[HPO_{4}^{-2}]}$

In general, ${{K}_{1}}>{{K}_{2}}>{{K}_{3}}$

The overall dissociation constant$(K)$ is given by the relation,

$K={{K}_{1}}\times {{K}_{2}}\times {{K}_{3}}$

(2) Dissociation constant for weak base : The equilibrium of $N{{H}_{4}}OH$ (a weak base) can be represented as,

$N{{H}_{4}}OH$  $\rightleftharpoons$ $NH_{4}^{+}+O{{H}^{-}}$

Applying the law of mass action, ${{K}_{b}}=\frac{[NH_{4}^{+}][O{{H}^{-}}]}{[N{{H}_{4}}OH]}$

${{K}_{b}}$ is constant at a definite temperature and does not change with the change of concentration.

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