Freezing Point Depression

When a solution freezes, the solid is usually pure solvent. Thus the solid-vapor equilibrium (sublimation) P-T curve is unaffected by the presence of solute. The intersection of this curve and the liquid-vapor curve is the triple point (nearly the same temperature as the freezing point, which is measured at atmospheric pressure). Since a solute lowers the solvent vapor pressure, the triple point is shifted to lower temperature, as shown in Figure 1.1. Detailed calculations show that the decrease in freezing point for a dilute solution is proportional to the total molal concentration of solutes

Kfp is the molal freezing-point constant of the solvent. Like K_bp, K_fp is a property of the solvent, independent of the nature of the solutes.

Example

The freezing point of pure camphor is 178.4°C and K_fp = 40.0 K kg/mol. Find the freezing point of a solution containing 1.50 g of a compound of molar mass 125 g/mol in 35.0 g of camphor. The molality of the solution is

Thus

Figure 1.1. Solid-liquid, solid-vapor and liquid-vapor equilibrium curves for pure water (solid curves) and for a solution (dashed curves). The triple point (where solid, liquid, and vapor coexist and at nearly the same temperature as the freezing point) is shifted to lower temperature for the solution.