The dissociation or association of solute molecules would alter the total number of particles present in the solution and hence affect the results of measured colligative properties. In such solutions, the value of the molar mass 0 he solute determined using colligative properties would be different from the actual molar mass, and it is called abnormal molar mass.

For an ideal dilute solution, the properties, namely, relative lowering of vapour pressure, elevation of boiling point, depression in freezing point and osmotic pressure do not depend on the chemical nature of the solute but depends only on the number of solute· particles (ions/molecules) present in the solution. These four properties are known as colligative properties.

Van't Hoff found out that for dilute solutions, the osmotic pressure is directly proportional to the molar concentration ofthe solute and the temperature of the solution. He proposed the following equation to calculate osmotic pressure which is now called as van't Hoff equation.
\(\pi\) = cRT
Here, 
c = Concentration of the solution in molarity
T = Temperature
R = Gas constant

A solution of potassium chloride in water deviates from ideal behavior because the solute dissociates to give K⁺ and CI^- ion which form strong ion-dipole interaction with water molecules.
\(\mathrm{KCl}_{(3)}+\mathrm{H}_{2} \mathrm{O}(l) \rightarrow \mathrm{K}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq})\)

In solution, acetic acid exists as a dimer by forming intermolecular hydrogen bonds, and hence deviates from Raoult's law.