Why do you get sometimes abnormal molecular mass of substances by using colligative properties of the solution? State the factors with examples which produces abnormality in the result.

Explanation

Colligative properties depend upon the molar concentration of solute in solution. These properties depend upon the number and not upon the nature of solute particles in the solution. In many cases solute dissociate or associate in solution. Abnormal values for colligative properties are observed. Hence, abnormal molecular mass is obtained.
(i) Dissociation : The solute particles dissociate into smaller ions. As a result the number of solute particles increase in solution. Therefore, colligative property increases.
For example, in a dilute solution chloride, whole of sodium chloride dissociates into `Na^(+)andCl^(-)` ions. Thus, observed value of the colligative property increases.
`NaCloverset("Water")rarrNa^(+)("aq")+Cl^(-)("aq")`
Molecular mass is inversely proportional to the colligative property. Since, colligative property increases, the molecular mass will decrease.
(ii) Association : The solute particles join together to form a bigger molecule. The number of solute particles decreases in solution on association. Thus, observed values of colligative properties will be less than the calculated value.
For example,
The molecules of benzoic acid associated in benzene to form dinners. Dimerisation occurs due to H-bonding. Its observed mass is found to be more than the calculated mass. For different solutes of same molar concentration, the colligative properties have the greater value for the solution which gives more number of particles on ionisation. For different solutions of same molar concentration of different non-eletrolyte solutes, the colligative properties have the same value for all.