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1.00 g of a non-electrolyte solute (molar mass 250 g mol⁻¹) was dissolved in 51.2 g of benzene. If the freezing point depression constant, Kf of benzene is 5.12 K kg mol⁻¹, the freezing point of benzene will be lowered by

1. 0.3 K
2. 0.5 K
3. 0.4 K
4. 0.2

Correct answer: 0.4 K

Solution

The freezing point depression (ΔTf) is calculated using the formula ΔTf = Kf × m, where m is the molality. Molality (m) = (moles of solute) / (mass of solvent in kg). Moles of solute = 1.00 g / 250 g mol⁻¹ = 0.004 mol. Mass of solvent = 51.2 g = 0.0512 kg. Molality = 0.004 / 0.0512 = 0.0781 mol kg⁻¹. ΔTf = 5.12 × 0.0781 = 0.4 K.

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