Assuming equal concentrations and complete dissociation, arrange these aqueous solutions by their freezing points. Highest freezing point
The Correct Answer and Explanation is :
When arranging aqueous solutions by their freezing points, it’s essential to consider the colligative properties of solutions, specifically freezing point depression. This property depends on the number of solute particles in the solution, not the chemical identity of the solute.
The key formula for freezing point depression is: ΔTf=i⋅Kf⋅m\Delta T_f = i \cdot K_f \cdot m
- ΔTf\Delta T_f: Freezing point depression
- ii: Van ‘t Hoff factor (number of particles the solute dissociates into)
- KfK_f: Freezing point depression constant for the solvent
- mm: Molality of the solution
For equal concentrations and assuming complete dissociation, the freezing point is primarily determined by the Van ‘t Hoff factor (ii), which is the total number of particles produced when the solute dissociates.
Example Solutions:
- NaCl: Dissociates into 2 particles (i=2i = 2) NaCl→Na++Cl−\text{NaCl} \to \text{Na}^+ + \text{Cl}^-
- K2_2SO4_4: Dissociates into 3 particles (i=3i = 3) K2SO4→2K++SO42−\text{K}_2\text{SO}_4 \to 2\text{K}^+ + \text{SO}_4^{2-}
- CaCl2_2: Dissociates into 3 particles (i=3i = 3) CaCl2→Ca2++2Cl−\text{CaCl}_2 \to \text{Ca}^{2+} + 2\text{Cl}^-
- Glucose (C6_6H12_{12}O6_6): Does not dissociate (i=1i = 1).
Freezing Point Order (Highest to Lowest):
- Glucose (i=1i = 1): Least freezing point depression.
- NaCl (i=2i = 2).
- K2_2SO4_4 and CaCl2_2 (i=3i = 3): Greatest freezing point depression (lowest freezing point).
Explanation:
Glucose has the highest freezing point because it does not dissociate, producing only 1 particle per molecule. NaCl dissociates into 2 particles, leading to greater freezing point depression than glucose. Both K2_2SO4_4 and CaCl2_2 dissociate into 3 particles, causing the most significant freezing point depression and having the lowest freezing point.
This demonstrates how the number of dissociated particles directly impacts the freezing point of solutions.