Which 0.1 molal aqueous solution will have the lowest freezing point?

• A. Al(NO3)3
• B. CaCl2
• C. C2H5OH
• D. NaCl

Answer: (A)

Freezing point depression is a colligative property: it depends on how many dissolved particles are present, not on their identity. The change in freezing point is ΔTf = i m Kf, where m is molality, Kf is the solvent’s freezing point constant, and i is the van’t Hoff factor, the number of particles produced per formula unit.

Here m is 0.1 for all solutions, and for water Kf is about 1.86 °C·kg/mol. The particle counts are: aluminum nitrate dissociates into four ions (Al3+ and 3 NO3−), so i ≈ 4; calcium chloride into three ions (Ca2+ and 2 Cl−), so i ≈ 3; sodium chloride into two ions, so i ≈ 2; ethanol does not dissociate, so i = 1. Therefore the freezing point depressions are roughly 0.74°C, 0.56°C, 0.37°C, and 0.19°C, respectively.

The largest depression means the lowest freezing point, so the aluminum nitrate solution has the lowest freezing point.