# Why does an 0.100 m solution of HCl dissolve in benzene have a freezing point depression of 0.512°C, while an 0.100 m solution of HCl in water has a freezing point depression of 0.372°C?

Jul 19, 2014

The reason is that HCl ionizes in water but not in benzene.

Freezing point is a colligative property. It depends on the number of particles in solution.

ΔT_"f" = iK_"f"m

In benzene

Benzene is a nonpolar solvent, so the HCl remains as molecules in the solution.

HCl(g) → HCl(in benzene solution)

Thus, 1 mol of HCl gas gives 1 mol of HCl molecules in solution. $i$ = 1. So

ΔT_"f" = 1 × 5.12 °C·kg·mol⁻¹ × 0.100 mol·kg⁻¹ = 0.512 °C

In water

HCl reacts with water to form hydronium ions and chloride ions.

HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq)

Thus, 1 mol of HCl gives 2 mol of particles in solution. $i$ = 2.

ΔT_"f" = 2 × 1.86 °C·kg·mol⁻¹ × 0.100 mol·kg⁻¹ = 0.372 °C