Association of Solute Molecules

Certain solutes in solution are found to associate. This eventually leads to a decrease in the number of molecular particles in the solution.

Thus, it results in a decrease in the values of observed colligative properties. As discussed, the colligative properties are inversely related to the molecular mass.

Therefore higher values are obtained for molecular masses than normal values for unassociated molecules.

For e.g., acetic acid dissolved in benzene shows a molecular mass of 120 (normal molecular mass is 60). Similarly, benzoic acid dissolved in benzene is found to have molecular mass double of its normal molecular mass. This is explained by the fact that both the acetic acid and benzoic acid form dimers in solution due to hydrogen bonding.