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On dissolving 0.5 g of a non-volatile non-ionic solute to 39 g of benzene, its vapor pressure decreases from 650 mm Hg to 640 mm Hg. The depression of freezing point of benzene (in K) upon addition of the solute is ____.
(Given data: Molar mass and the molal freezing point depression constant of benzene are 78 g mol⁻¹ and 5.12 K kg mol⁻¹, respectively)
- 1.0256
- 1.026
- 1.03
- 1.04
Correct answer: 1.0256
Solution
Relative lowering (650-640)/650=10/650 equals mole fraction of solute; with 0.5 mol benzene this gives solute molar mass 64 and molality 0.2003 mol/kg. Then dTf=5.12*0.2003=1.0256 K, so the answer is 1.0256, not 1.04.
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