‘Assuming equal concentrations and complete dissociation, rank these aqueous solutions by their freezing points_ Highest freezing point Lowes freezing point NHACI Li;PO4 NazSOa’
The Correct Answer and Explanation is:
To determine the freezing points of aqueous solutions of NH₄Cl, Li₃PO₄, and Na₂SO₄, we need to consider freezing point depression, which depends on the van’t Hoff factor (i)—the number of particles into which a solute dissociates in solution. The more particles produced, the greater the freezing point depression, and thus the lower the freezing point.
Dissociation of the Compounds:
- NH₄Cl (Ammonium chloride)
Dissociates into: NH₄⁺ + Cl⁻
i = 2 - Na₂SO₄ (Sodium sulfate)
Dissociates into: 2Na⁺ + SO₄²⁻
i = 3 - Li₃PO₄ (Lithium phosphate)
Dissociates into: 3Li⁺ + PO₄³⁻
i = 4
Freezing Point Depression (ΔTf):
The freezing point depression is calculated using the formula:
ΔTf = i × Kf × m
Where:
- i is the van’t Hoff factor
- Kf is the cryoscopic constant (same for water in all cases)
- m is the molality (assumed equal for all solutions)
Since all solutions have the same concentration and solvent, ΔTf is directly proportional to i.
Ranking (Highest to Lowest Freezing Point):
- NH₄Cl (i = 2) → Highest freezing point
- Na₂SO₄ (i = 3) → Intermediate freezing point
- Li₃PO₄ (i = 4) → Lowest freezing point
Explanation:
Freezing point depression occurs because the presence of solute particles interferes with the formation of a solid structure, requiring a lower temperature to freeze. More dissociated ions lead to a greater effect. Although all solutes are ionic and fully dissociate, they produce different numbers of ions per formula unit. NH₄Cl produces 2 ions, Na₂SO₄ yields 3, and Li₃PO₄ gives 4. Therefore, Li₃PO₄ causes the largest depression and has the lowest freezing point. Conversely, NH₄Cl causes the least depression, so it has the highest freezing point.
Final order: NH₄Cl > Na₂SO₄ > Li₃PO₄
