Certain compounds when dissolved in suitable solvents either dissociate or associate.
For example ethanoic acid dimerises in benzene due to hydrogen bonding, while in water, it dissociates and forms ions. As a result the number of chemical species in solution increases or decreases as compared to the number of chemical species of solute added to form the solution. Since the magnitude of colligative property depends on the number of solute particles, it is expected that the molar mass determined on the basis of colligative properties will be either higher or lower than the expected value or the normal value and is called abnormal molar mass.
In order to account for the extent of dissociation or association of molecules in solution, Van’t Hoff introduced a factor, i, known as the Van’t Hoff factor. It can be defined as follows.
i = Expected molar mass/Abnormalmolar mass
= Observed colligative property/Calculated colligative property
= Total number of of particles after association/dissociation/Number of of particles before association/dissociation
