1. Why do we get abnormal molecular masses from colligative properties? What is Van’t Hoff factor? How will you explain cases of association and dissociation of solute in such cases?

i) Colligative properties depend upon the molar concentration of the solute in solution. These properties depend upon the number and not upon the nature of the solute particles in solution. Therefore, in many cases where the solute associate or dissociate in solution, certain abnormal values of colligative properties are obtained. The values of molecular masses calculated based on colligative properties in such cases will also be abnormal.


ii) Association of solute particle : 


If a solute associates in the solution, the number of particles present in the solution will be less than the actual number dissolved and hence the value of osmotic pressure and other colligative properties will be smaller and consequently the molecular mass indicated will be higher than the true molecular mass. For example, carboxylic acids such as acetic acid and benzoic acid associate in benzene to form dimers due to hydrogen bonding.


iii) Dissociation :

A number of electrolytes dissociate in solution to give two or more particles (ions). Therefore, the number of solute particles, in solutions of such substances, is more than the expected value. Accordingly, such solutions exhibit higher values of colligative properties. Since colligative properties are inversely proportional to molecular masses, therefore, molecular masses of such substances as calculated from colligative properties will be less than their normal values.

iv)  VanntHoff factor is a defined as the effect of a solute upon colligative properties such as.

1. Osmotic pressure 

2. Relative lowering in vapor pressure, 

3. Elevation of boiling point 

4. Freezing point depression

thus its is the ratio of  the actual concentration of solute present in solution produced with respect to the concentration of a substance as  calculated from its mass theoretically.



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