The measure of the effect of a solute upon the colligative properties of a solution is known as the Van't Hoff factor.
Mathematically, the ratio between the actual concentration of particles produced when the substance is dissolved and the concentration of a substance as calculated from its mass is the Van't Hoff factor.
If we consider the electrolytes than the Van't Hoff factor should be equal to the number of discrete ions in a formula unit of the substance, however, this is not true and the van't Hoff factor is always less than the expected value as ion-pairing takes place in the solutions.
Ion pairing takes place when ions of opposite electrical charge come together in solution and form a distinct chemical entity.
The van't Hoff factor is defined as the ratio between the actual concentration of particles when are in a dissolved state in comparison to the theoretical concentration calculated from its mass. The observed van't Hoff factor for an electrolyte is less than the expected factor because of the ion pairing effect. Ion pairing is the association of ions because of having opposite charges. When ion pairing takes place in a solution, two ions association would apparently be one, which reduces the observed van't Hoff factor in comparison to the theoretical.
