question_answer 115)

  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:

                  Certain compounds when dissolved in suitable solvents either dissociate or associate. e.g., ethanoic acid dimerises in benzene due to H-bonding, while in water, it dissociates and forms ions.                                            As a result the number of chemical species in solution increases 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, it is       expected that the molar mass determined on the basis of colligative properties will either higher or lower than the expected value or the normal 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, known as the van't Hoff factor.