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The freezing point of benzene decreases by 0.45 °C when 0.2 g of acetic acid is added to 20 g of benzene. If acetic acid associates to form a dimer in benzene, percentage of association of acetic acid in benzene will be: (Kf for benzene = 5.12 K kg mol−1)

1. 64.6%
2. 80.4%
3. 74.6%
4. 94.6%

Correct answer: 94.6%

Solution

m = (0.2/60)/0.020 = 0.1667 mol/kg. i = dTf/(Kf*m) = 0.45/(5.12*0.1667) = 0.527. For dimerization i = 1 - alpha/2 -> alpha = 2(1-0.527) = 0.945, i.e. about 94.6%.

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