Dissociation constants of acids and Bases

Dissociation constants of acids and Bases

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

Applying the law of mass action,

Where, is the dissociation constant of the acid, . 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.

(I step)

(II step)

(III step)

Let and be the ionization constants of first, second and third steps respectively. Thus,

;;

In general,

The overall dissociation constant is given by the relation,

(2) Dissociation constant for weak base: The equilibrium of (a weak base) can be represented as,

Applying the law of mass action,

is constant at a definite temperature and does not change with the change of concentration.

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