Question

Why is the mass determined by measuring a colligative property in case of some solutes abnormal ? Discuss it with the help of Van’t Hoff factor.

Answer 1

Certain solutes/compounds when dissolved in suitable solvents either dissociate or associate.

For example: (i) Ethanoic acid dimerises in benzene due to hydrofen bonding, while

(ii) In water, it dissociates and forms ions.

As a result, the number of chemical species in solution incerases or decreases as compared to the number of chemical species of solute added to form the solution. Since the magnitude of colligative property depends on the number of solute particles relative to the total number of particles in solution, it is expected that the molar mass determined on the basis of colligative properties will be either higher or lower than the expected value or the bormal value and is called abnormal molar mass.

In order to account for the extent of dissociation or association of molecules in solution, Van't Hoff introduced a factor, i know as the van't Hoff factor. It can be defined as follows.

i = `"Expected molar mass"/"Abnormal molar mass"`

= `"Observed colligative property"/"Calculated colligative property"`

= `"Total number of moles of particles after assocaition/dissociation"/"Number of moles of particles before assciation/dissociation"`

Here abnormal molar mass is the experimentally determined molar mass and calculated colligative properties are obtained by assuming that the non-volative solute is neither associated nor dissociated in case of association the value of i(van't Hoff factor) is lesser than unity while for dissociation iti is greater than unity.