194.

Discuss the ionisation of weak electrolytes (Ostwald’s dilution law).

As the weak electrolytes are ionised to a small extent, there is a state of equilibrium between the unionised electrolyte and the ions formed in solution. To illustrate.this, let us consider a weak electrolyte HA to be dissolved in water, a be the degree of ionisation and C be the molar concentration of HA.
                                  HA     H⁺(aq) + A^-(aq)
Initial  conc.                C            0               0
Equilibrium conc.                         
Applying law of chemical equilibrium,

                 

where K_a is ionisation (or dissociation) constant of the weak acid HA.



This equation is often referred to as Ostwald’ dilution law equation since the degree of dissociation of a weak electrolyte is very small as compared to unity.

Thus, the degree of ionisation of a weak electrolyte is inversely proportional to the square root of the molar concentration of the solution of the electrolyte.

Hence the degree of ionisation of weak electrolyte goes on increasing with the decrease in molar concentration i.e. with dilution and it reaches the maximum value (unity) in very dilute solution. Thus, all weak electrolytes undergo almost complete ionisation at infinite dilution.
For a weak base BOH:
Let us consider a weak base BOH dissolved in water, a be the degree of ionisation and C be the molar concentration of BOH.

                         
Initial conc.           C              0                 0
Equilibrium                                  
conc. 
Applying law of chemical equilibrium,