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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 association of acetic acid in benzene will be (Kf for benzene = 5.12 K kg mol⁻¹)

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

Correct answer: 94.6%

Solution

The correct option is right because the observed freezing point depression indicates that the effective number of solute particles is less than expected due to dimerization. By calculating the degree of association using the freezing point depression formula and the van 't Hoff factor for the dimer, we find that approximately 94.6% of acetic acid associates in benzene.

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