Van't Hoff's factor (i) : In the year 1886, Van't Hoff introduced the factor 'i' called Van't Hoff's factor, to express the extent of the association or dissociation of the solutes in solution. It is the ratio of the normal and observed molecular masses of solute, that is

 


In the case of association, observed molecular mass being more than normal, the factor i has a value less than 1. But in case of the dissociation, the Van't Hoff's factor is more than 1 because observed molecular mass has a lesser value than the normal molecular mass. In case there is no dissociation value of 'i' becomes equal to 1.

As colligative properties are inversely proportional to the molecular masses, the Van't Hoff's factor can also be written as follows
,

The introduction of the Van't Hoff factor modifies the equations for the colligative properties as written below,

Relative lowering of vapour pressure

Elevation of boiling point, T_b = ik_bm

Depression in freezing point, T_f = ik_fm

Osmotic pressure,   

From value of 'i', it is possible to calculate the degree of dissociation or the degree of association of substance.

The degree of dissociation (a): It can be defined as the fraction of the total molecules which dissociate into the simpler molecules or ions.

; m= number of particles in the solution

The degree of association (a): It can be defined as the fraction of the total number of molecules which associate or combine together resulting in formation of the bigger molecules.

; m = number of particles in the solution.

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