Which one of the following 0.06 M aqueous solutions has lowest freezing point?

Solution

Chemistry ma’am · 3 steps

Same concentration everywhere, so just count the ions each solute gives: more particles, lower freezing point.

1. What decides it. Δ T_f = i K_f m, and K_f is the same for all four because the solvent is water. At equal concentration, the solute with the largest van't Hoff factor i lowers the freezing point the most.

   Solute	Particles in water	i (ideal)
   Al₂(SO₄)₃	2Al³⁺ + 3SO₄²⁻	5
   C₆H₁₂O₆	stays as whole molecules	1
   KI	K⁺ + I⁻	2
   K₂SO₄	2K⁺ + SO₄²⁻	3

2. Compare. The effective particle concentration is i × 0.06: 0.30 for Al₂(SO₄)₃, against 0.18, 0.12 and 0.06 for the others. The biggest Δ T_f means the freezing point falls furthest below 0 °C.
3. So. Al₂(SO₄)₃, with i = 5, has the lowest freezing point. The answer is (a).

The idea to keep. Colligative properties count particles, not their identity. At equal concentration in the same solvent, rank the solutes by i: the largest i gives the largest Δ T_f and the lowest freezing point.

Answer from the NTA final answer key. Solution by PrepWiser.