Expression of Ostwald’s dilution law in the case of a weak electrolyte :
Consider the dissociation of a weak electrolyte BA. Let V dm3 of a solution contain one mole of the electrolyte. Then the concentration of a solution is, C =1/V mol dm-3. Let α be the degree of dissociation of the electrolyte.
Applying the law of mass action to this dissociation equilibrium, we have,
As the electrolyte is weak, α is very small as compared to unity,
∴ (1 – α) ≈ 1.
This is the expression of Ostwald’s dilution law. Thus, the degree of dissociation of a weak electrolyte is directly proportional to the square root of the volume of the solution containing 1 mole of an electrolyte.
