Calculate ΔTf for a solution with 2.0 m of nonvolatile solute in a solvent with Kf = 1.86 °C/m.

Calculate ΔTf for a solution with 2.0 m of nonvolatile solute in a solvent with Kf = 1.86 °C/m.

• A. 3.72 °C
• B. 1.86 °C
• C. 7.44 °C
• D. 0.93 °C

Answer: (A)

Freezing point depression is a colligative property: it depends on the number of solute particles in the solvent. For a nonvolatile solute that doesn’t dissociate, the van’t Hoff factor i is about 1, so ΔTf = i × Kf × m ≈ Kf × m. With Kf = 1.86 °C/m and a molality of 2.0 m, you get ΔTf = 1.86 × 2.0 = 3.72 °C. The solution’s freezing point is lowered by 3.72 °C (Tf(solution) = Tf(solvent) − 3.72 °C).

Freezing point depression is a colligative property: it depends on the number of solute particles in the solvent. For a nonvolatile solute that doesn’t dissociate, the van’t Hoff factor i is about 1, so ΔTf = i × Kf × m ≈ Kf × m. With Kf = 1.86 °C/m and a molality of 2.0 m, you get ΔTf = 1.86 × 2.0 = 3.72 °C. The solution’s freezing point is lowered by 3.72 °C (Tf(solution) = Tf(solvent) − 3.72 °C).