Assuming equal concentrations and complete dissociation, arrange these aqueous solutions by their freezing points. Highest freezing point Lowest freezing point Answer Bank FeCl3 NaBr Al2(SO4)3 Li?SO?
The Correct Answer and Explanation is:
Correct Order (from Highest to Lowest Freezing Point):
NaBr > Li₂SO₄ > FeCl₃ > Al₂(SO₄)₃
Explanation
The freezing point of a solution depends on freezing point depression, a colligative property that is influenced by the number of solute particles in the solution. The more particles produced upon dissociation, the lower the freezing point. This relationship is given by:
ΔTf = i × Kf × m
Where:
- ΔTf is the freezing point depression
- i is the van’t Hoff factor (number of ions the compound dissociates into)
- Kf is the cryoscopic constant (depends on the solvent, here water)
- m is the molality of the solution
Assuming equal concentrations and complete dissociation, the key factor is the van’t Hoff factor (i) for each compound:
- NaBr → Na⁺ + Br⁻ → i = 2
- Li₂SO₄ → 2Li⁺ + SO₄²⁻ → i = 3
- FeCl₃ → Fe³⁺ + 3Cl⁻ → i = 4
- Al₂(SO₄)₃ → 2Al³⁺ + 3SO₄²⁻ → i = 5
Because all the solutions have the same molal concentration and completely dissociate, the one that produces the fewest ions (smallest i) will have the least freezing point depression, meaning the highest freezing point.
- NaBr (i = 2) will have the highest freezing point
- Li₂SO₄ (i = 3) is next
- FeCl₃ (i = 4) follows
- Al₂(SO₄)₃ (i = 5) will have the lowest freezing point
Thus, as the number of ions increases, the freezing point drops further. This trend illustrates the direct connection between ion concentration from dissociation and the extent of freezing point depression.
