Assuming equal concentrations and complete dissociation, match these aqueous solutions to their relative freezing points. Na2SO4 Highest freezing point v KI Choose… Li3PO4 Choose…
The Correct Answer and Explanation is :
To match these aqueous solutions to their relative freezing points, we need to consider the van’t Hoff factor (ii), which represents the number of particles into which a compound dissociates in solution. The freezing point depression is directly proportional to ii, so the greater the number of dissolved particles, the lower the freezing point of the solution.
Step 1: Calculate ii for each compound
- Na2_2SO4_4: Sodium sulfate dissociates into three ions: Na2SO4→2Na++SO42−\text{Na}_2\text{SO}_4 \rightarrow 2\text{Na}^+ + \text{SO}_4^{2-} Here, i=3i = 3.
- KI: Potassium iodide dissociates into two ions: KI→K++I−\text{KI} \rightarrow \text{K}^+ + \text{I}^- Here, i=2i = 2.
- Li3_3PO4_4: Lithium phosphate dissociates into four ions: Li3PO4→3Li++PO43−\text{Li}_3\text{PO}_4 \rightarrow 3\text{Li}^+ + \text{PO}_4^{3-} Here, i=4i = 4.
Step 2: Freezing point depression relationship
The freezing point depression is given by: ΔTf=i⋅Kf⋅m\Delta T_f = i \cdot K_f \cdot m
Since all solutions are at equal concentrations and KfK_f is constant for water, the relative freezing points depend only on ii:
- Higher ii: Greater freezing point depression (lower freezing point).
- Lower ii: Smaller freezing point depression (higher freezing point).
Step 3: Rank freezing points
- i=2i = 2 (KI) → Highest freezing point.
- i=3i = 3 (Na2_2SO4_4) → Intermediate freezing point.
- i=4i = 4 (Li3_3PO4_4) → Lowest freezing point.
Final Answer:
- Na2_2SO4_4: Intermediate freezing point.
- KI: Highest freezing point.
- Li3_3PO4_4: Lowest freezing point.
Explanation:
The freezing point depression depends on the number of dissolved particles (ii). KI produces the fewest particles (i=2i=2), leading to the least depression and the highest freezing point. Li3_3PO4_4 produces the most particles (i=4i=4), resulting in the greatest depression and the lowest freezing point. Na2_2SO4_4 lies in between with i=3i=3.