moles Degree of dissociation of CH COOH α Number of moles at equilibrium - α α α Equilibrium concentration ( - ) C α α C α C Substituting the equilibrium concentration in equation ( . ) K = ( C)( C) ( - )C K = - a a α α α α α .....( . ) We know that weak acid dissociates only to a very small extent. Compared to one, α is so small and hence in the denominator ( - ) .
α The above expression ( . ) now becomes, K = = K = K a a a α α α ⇒ .....( . ) Let us consider an acid with K a value - and calculate the degree of dissociation of that acid at two different concentration M - and M - using the above expression ( . ) For M - , XII U8-Ionic XII U8-Ionic - - - - α = = = = .
- - - - For M - acid, α = = - - i.e, When the dilution increases by times, (Concentration decreases from M - to M - ), the dissociation increases by times. Thus, we can conclude that, when dilution increases, the degree of dissociation of weak electrolyte also increases. This statement is known as Ostwald’s dilution Law. The concentration of H (H O ) + can be calculated using the K a value as below.
[ H ]= C α (Refer table) .....( . ) Equilibrium molar concentration of [H ] + is equal to α C ∴ [H ]= K K C [H ] = K C a a a .....( . ) Similarly, for a weak base K = C and = K [OH ] = C (or) [OH ]= K C b b b α α α .....( . ) Example .
A solution of .10M of a weak electrolyte is found to be dissociated to the extent of . % at