Abnormal Molecular Masses

The relation between colligative property and molecular mass of the solute is based on the following assumptions:

i) The solution is dilute so that Raoult's law is obeyed.

ii) The solute neither undergoes dissociation nor association in solution.

When the molecular mass of unknown solute is measured using colligative properties, the cases where these assumptions are not valid, certain discrepancies between observed and calculated values of colligative properties are noticed.

The anomalies are primarily due to:

(i) Association of solute molecules and

(ii) Dissociation of the solute particles.