Which of the following 0.10 M aqueous solutions will have the lowest freezing point? Assume complete dissociation for strong electrolytes.
Show the answer and explanation
Correct answer
D. AlCl3
Principle or equation
Freezing point depression is a colligative property: ΔTf = i * Kf * m. The solution with the highest van't Hoff factor (i) will have the lowest freezing point.
Why this answer is correct
Van't Hoff factors: NaCl → i=2, CaCl2 → i=3, C6H12O6 → i=1 (non-electrolyte), AlCl3 → i=4. Since all have the same molality, AlCl3 produces the most particles, causing the greatest freezing point depression, hence the lowest freezing point.
Example
For 0.10 M solutions, the freezing point depression is proportional to i. For CaCl2, i=3, so ΔTf is three times that of glucose.
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