Such a molar mass
that is either lower or higher than the expected or normal value is called
as abnormal molar mass In 1880 van’t Hoff introduced a factor i, known as the van’t Hoff
factor, to account for the extent of dissociation or association This
factor i is defined as:
Normal molar mass
Abnormal molar mass

i
  
Observed colligative property
Calculated colligative property

Total number of moles of particles after association/dissociation
Number of moles of particles before association/dissociation

i
Here abnormal molar mass is the experimentally determined molar
mass and calculated colligative properties are obtained by assuming
that the non-volatile solute is neither associated nor dissociated In
case of association, value of i is less than unity while for dissociation it
is greater than unity