CBSE Class 12-science Answered

Van't Hoff factor is a measure of the effect of a solute upon colligative properties such as osmotic pressure, relative lowering in vapor pressure, elevation of boiling point and freezing point depression. The van 't Hoff factor is 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. For most non-electrolytes dissolved in water, the van' t Hoff factor is essentially 1. For most ionic compounds dissolved in water, the van 't Hoff factor is equal to the number of discrete ions in a formula unit of the substance. This is true for ideal solutions only, as occasionally ion pairing occurs in solution. At a given instant a small percentage of the ions are paired and count as a single particle. Ion pairing occurs to some extent in all electrolyte solutions. This causes deviation from the van 't Hoff factor. The deviation for the van 't Hoff factor tends to be greatest where the ions have multiple charges.

The van’t Hoff factor is a relation between the ideal value of a solution’s colligative properties and the observed colligative properties.  The number of moles of particles in the solution in relation to the moles of solute dissolved will always be lower than the idea for ionic solutes.  The ideal values for the ionization of  sodium chloride is two since there is one Na⁺ and one Cl^- for each NaCl that is dissolved 1 + 1 = 2, for magnesium chloride the ideal value is three or one Mg⁺ and two Cl^-, 1 + 2 = 3. 

The van’t Hoff factor is observed slightly less than it is calculated.