Question-22 Which of the following aqueous solutions exhibits lowest freezing point depression assuming complete dissociation?
(A) 0.1\ m\ NaCl
(B) 0.05\ m\ MgSO_4
(C) 0.08\ m\ AlPO_4
(D) 0.06\ m\ Al_2(SO_4)_3
Answer : (B)
Explanation:
Depression of freezing point is colligative property which depends on number of particles present in solution. Lesser is the number of particles, less will be the depression.
| Solution | Dissociation | Molality of ions after dissociation |
|---|---|---|
| 0.1 m NaCl | NaCl \rightarrow Na^+ + Cl^- | 0.1 × 2 = 0.2 m |
| 0.05 m MgSO₄ | MgSO_4 \rightarrow Mg^{2+} + SO_4^{2-} | 0.05 × 2 = 0.1 m |
| 0.08 m AlPO₄ | AlPO_4 \rightarrow Al^{3+} + PO_4^{3-} | 0.08 × 2 = 0.16 m |
| 0.06 m Al₂(SO₄)₃ | Al_2(SO_4)_3 \rightarrow 2Al^{3+} + 3SO_4^{2-} | 0.06 × 5 = 0.3 m |