A solution that contains 3.00 g of urea [CO(NH₂)₂] dissolved in 78.1 g of benzene (C₆H₆) has a freezing point of 2.37 ° C. The normal freezing point of benzene is 5.51 ° C. What is the freezing point depression constant for this solvent?

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The freezing point depression (ΔTf) is calculated using the formula ΔTf = i * Kf * m, where i is the van't Hoff factor (1 for urea), Kf is the freezing point depression constant, and m is the molality. The change in freezing point is 5.51 °C - 2.37 °C = 3.14 °C. The molality (m) is calculated as moles of solute per kg of solvent. Moles of urea = 3.00 g / 60.06 g/mol = 0.0499 mol. Molality = 0.0499 mol / 0.0781 kg = 0.639 m. Solving for Kf gives Kf = ΔTf / m = 3.14 °C / 0.639 m = 4.91 °C/m, which is closest to 4.90 °C/m.

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