Daily answer writing · Thu, 12 Nov, 2026
Abnormal molar mass: benzoic acid dimerises in benzene, K₂SO₄ dissociates in water (van 't Hoff factor)
(a) 1.00 g of benzoic acid dissolved in 50.0 g of benzene lowers the freezing point by 0.48 K. Calculate the apparent molar mass and the degree of dimerisation of benzoic acid ($K_f$ of benzene = 5.12 K kg mol−1; C6H5COOH = 122.12 g mol−1).
(b) A 0.100 mol kg−1 aqueous solution of K2SO4 freezes at −0.43 °C. Calculate the van 't Hoff factor and the apparent degree of dissociation ($K_f$ of water = 1.86 K kg mol−1). Why is this less than 100% though K2SO4 is a strong electrolyte?
Syllabus: Common to CSE Paper I, IFoS Paper I and Geo-Scientist Mains (physical): solutions, colligative properties, depression of freezing point, abnormal molar masses, van 't Hoff factor, association and dissociation.
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