Why do we get sometimes abnormal molecular masses of the substances by using colligative properties of the solutions ? State the factors with suitable examples which bring abnormally in results thus obtained.

Explanation

Abnormal molecular masses. The molecular masses of certain substances as determined experimentally by osmotic pressure, elevation in boiling points, depression in freezing points or relative lowering in vapour pressure are different from those calculated theoretically from their chemical formulae. In some cases, the determined molecular masses are high while in other they are less as compared to their normal values. This may be due to the association or the dissociation of the solute molecules in the solution. Therefore, the number of the molecules undergo a change. As the colligative properties depend only upon the number of the particles, they are expected to show certain deviations. These two changes are being discussed below:
1. Association of solute particles. There are certain substances whose molecules undergo association when dissolved in a solvent i.e., two, three or even more molecules exist in combination with each other to form bigger molecules. For example, suppose n simple molecules combine to form an associated molecule as :
` nA = A_n `
n single molecules Associated molecule
Therefore, the total number of molecules in solution become less than the number of molecules of the substances added and, therefore, colligative properties are inversely proportional to the molecular mass of the solute, the molecular masses in such cases will be greater than the theoretical values. For example, in benzene solvent, benzoic acid exists as dimer as:
`2C_6 H_5 COOH = (C_6 H_5 COOH)_2`
The molecular mass has been found to be nearly 244 which is about double than normal values is 122. The association of solute molecules in a solution is generally due to the hydrogen bonding between these molecules as shown below:

2. Dissociation of solute molecules. There are certain substances which dissociate in a solvent to give two or more particles. For example, KCl dissociates to give `K^+ and Cl^-` ions.
Consequently, the total number of particles increases in solution and therefore, the colligative properties of such solution will be large. Since colligative properties are inversely proportional to molecular mass, the observed molecular mass will be less than the theoretical value. The normal or theoretical molecular mass of KCI is 74.5. However, the depression in freezing point method has given the molecular mass of KCl as 40.5. Consequently, there is difference in the values of the molecular masses.