CBSE Class 12-science Answered

If the molecular mass of a substance determined by studying any of the colligative properties comes out to be different than the theoratically expected value, then the substance is said to show abnormal molecular mass.

The various relations for the colligative properties hold good only for dilute solutions which act as ideal solutions. Thus, even when the solute is non-volatile, non-electrolytic, non-associated and non-dissociated, deviations from the theoratically expected values of colligative properties and molecular masses are observed at higher concentrations. This is because at higher concentrations, the moleculaes are so close to each other that they exert appreciable intermolecular forces of attraction resulting into the deviations from ideal behaviour.

Abnormal molecular masses are observed in any one of the following cases:

(a) When the solution is non-ideal, that is, the solution is not dilute.

(b) When the solute undergoes association in the solution.

(c) When the solute undergoes dissociation in the solution.

 

To calculate the extent of dissociation or association, van't Hoff in 1880 introduced a factor 'i' called van't Hoff factor.

van't Hoff factor is defined as the ratio of the experimental value of the colligative property to the calculated value of the colligative property that is van't Hoff factor,

 

 

Since the clligative proprty is inversely proportional to the molecular mass of the solute, we can also write