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

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

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Abnormal molecular masses are shown by the compounds which dissociate/associate in the solvent.

  1. Association: Colligative properties depend on the number of particles in a solution. Certain compounds like benzoic acid or ethanoic acid dimerise in benzene due to hydrogen bonding resulting in the reduction of the number of particles and thus, solutes show lower colligative property.
  2. Dissociation: Similarly, certain compounds like electrolytes (NaCl, KCl) etc dissociate into ions, increasing the number of particles, and thus, show higher value of colligative property.

Van’t Hoff introduced a factor to account for association or dissociation, known as Van’t Hoff factor.
i=\frac{\text{Expected molar mass}}{\text{Abnormal molar mass}}

=\frac{\text{Observed colligative property}}{\text{Calculated colligative property}}

=\frac{\text{Total number of moles of particles after association or disaasociation}}{\text{Total number of moles of particles before association or disaccioation}}

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