Question

Derive the expression of Ostwald’s dilution law in case of a weak acid (HA). 

OR 

Derive the relationship between degree of dissociation and dissociation constant of a weak acid.

Answer 1

Expression of Ostwald’s dilution law in case of a weak acid : 

Consider the dissociation of a weak acid HA. 

Let V dm³ of a solution contain one mole of weak acid HA. 

Then the concentration of a solution is, 

C = \(\frac{1}{V}\)mol dm^3-3. 

Let α be the degree of dissociation of HA.

Applying the law of mass action to this dissociation equilibrium, we have,

As the acid is weak, a is very small as compared to unity, 

∴ (1 – α) ≈ 1.

This is an expression for Ostwald’s dilution law. This shows that the degree of dissociation of a weak acid is directly proportional to the volume of solution containing one mole of acid or inversely proportional to square root of its concentration.