Abnormal Molar Masses

Question

Benzoic acid dissolved in benzene shows an abnormally high molar mass when measured by colligative property methods. Which phenomenon explains this, and what is the expected van't Hoff factor?

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Benzoic acid in benzene (a non-polar solvent) undergoes dimerisation via hydrogen bonding between the carboxylic acid groups of two molecules: 2C₆H₅COOH ⇌ (C₆H₅COOH)₂. This dimerisation reduces the number of solute particles in solution by half. Since colligative properties depend on the number of particles, a smaller number of particles leads to a smaller observed colligative effect than expected, giving an apparent molar mass that is twice the true value. The van't Hoff factor i = observed particles / expected particles = 0.5 (since 2 molecules become 1 dimer). Option A: Ionisation would give i > 1 (more particles), but benzoic acid ionises in polar solvents like water, not non-polar benzene. Option C: Solvation does not change the number of particles, and i = 2 is incorrect. Option D: Decomposition in benzene is not a known phenomenon for benzoic acid under normal conditions. This is a well-known NCERT example of association leading to abnormally high molar mass. Plausibility check: an elevated apparent molar mass (measured value ≈ 244 g/mol vs actual 122 g/mol) is consistent with dimerisation (factor of 2) and i = 0.5.

Abnormal Molar Masses

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