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?
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.
The formula for freezing point depression is
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.
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.