119. The molar mass of acetic acid determined from the measurement of a colligative property is greater than its formula molar mass. Explain why?

Answer:
The colligative properties of solution depend on the total number of solution.
since the eletrolytes ionise and give more than one particle per formula unit in solution the colligative effect of an eletrolyte solution is alaways greater than that of a non electrolyte of the same molar concentration.

The molar mass of acetic acid in a solution of benzene solvent is greater than its formula molar mass because two CH₃COOH molecules form a dimer (CH₃COOH)₂ in solution. But we know that the molar mass is defined as the mass of 6.023 x 10²³ particles of the substance. In case of a normal solute like CH₃COOH, the molar mass of acetic acid is equal to the mass of 6.023 x 10²³ molecules of formula CH₃COOH. While in solution the solution dimerises and becomes a bigger molecule like (CH₃COOH)₂. Now in this case the molar mass of the solute is equal to the mass of 6.023 x 10²³ units of formula (CH₃COOH)₂. Naturally, this molar mass is larger than the normal formula molar mass. In case of association, Van’t Hoff factor is less than 1, therefore,

M_B (abnormal) > M_B (normal) when i < 1.