Mon February 13, 2012 By: Aarushi -

Why do KCl and acetic acid show abnormal molar mass? What do we infer when Vant HoffÂ’s factor is unity.

Expert Reply
Tue February 14, 2012
KCl dissociates into K+ and Cl? ions when dissolved in water. So the number of solute particles in its solution would be double the number of particles if no dissociation had taken place. Hence it is expected to have molecular mass (on the basis of colligative properties) equal to half of its normal molecular mass, i.e., 74.5/2 = 37.25. However, the molecular mass of KCl is found to be 40.3 by studies of depression in freezing point. The difference in two values is due to the fact that there are strong attractive forces present in the oppositely changed ions of the strong electrolyte in solution. These electrical forces hold together a number of the ion pairs. Thus such electrolytes are are completely dissociated and number of ions formed in solution is not exactly the double but is some what less. Consequently, there is difference in the values of the molecular masses.
Acetic acid dissolved in benzene shows a molecular mass of 120 (normal molecular mass = 60).  This is explained by the fact that acetic acid forms dimers in solution due to hydrogen bonding.

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