Abnormal Colligative Properties

The colligative properties of the solutions depend only upon the number of solute particles present in the solution and not on their nature. But sometimes while measuring colligative properties abnormal results are obtained due to the following reasons:

1. If the solution is very concentrated, the particles of the solute start interacting with each other. Therefore, the solution should not be concentrated.
2. In case of association two or more solute molecules associate to form a bigger molecule, the number of effective molecules in the solution decreases. Since, the colligative property is inversely proportional to the molar mass, the molar mass of such solutes calculated on the basis of collgative property will be greater than the true molar mass of the solute.
3. In case of dissociation of the solute in the solution, the number of effective solute particles increases. In such cases the value of the observed collgative property will be greater than that calculated on the basis of undissociated solute particles. The molar mass of the solute calculated from the measurement of collgative property will be lower than the true molar mass of the solute.

Van’t Hoff Factor

In order to account for extent of association or dissociation, Van’t Hoff introduced a factor 'i'.

Degree of Association

Degree of association is defined as the fraction of the total number of molecules which associate to form a bigger molecule.

Degree of Dissociation

Degree of dissociation is defined as the fraction of the total number of particles that dissociate, i.e., break into simpler ions.