Question

Why do we get abnormal molecular masses of the substances using colligative properties of solutions only sometimes? What are the nature of these abnormalities? 

Answer 1

We get abnormal molecular masses using colligative properties only sometimes due to the association or dissociation of solute particles in solution. Colligative properties like osmotic pressure, boiling point elevation, freezing point depression, and vapour pressure lowering depend on the number of solute particles, not their nature. When this number changes due to association or dissociation, the calculated molecular mass becomes abnormal (i.e., different from the expected or theoretical value).