For a 0.10 m NaCl solution in water, where i ≈ 2 and Kf = 1.86°C·kg/mol, what is the freezing point depression ΔTf?

• A. 0.26°C
• B. 0.37°C
• C. 0.19°C
• D. 0.74°C

Answer: (B)

Freezing point depression depends on how many particles are in the solution. The formula is ΔTf = i × Kf × m, where i is the van’t Hoff factor and m is molality. For NaCl, it dissociates into two ions in water, so i ≈ 2. The solution is 0.10 m, meaning 0.10 mol of NaCl per kilogram of solvent, so m = 0.10. Plugging in: ΔTf = 2 × 1.86 × 0.10 = 0.372°C, which rounds to 0.37°C. This is the decrease in freezing point, so the solution freezes about 0.37°C below pure water. The value assumes ideal behavior; slight deviations can occur if ion pairing becomes significant, but 0.37°C is the standard estimate for a dilute NaCl solution.