Question

Describe cryoscopic method to determine the molecular mass of a non volatile solute.

Answer 1

To determine the molecular mass of a non volatile solute, the formula used is ${M_B} = \frac{{{K_f} \times {W_B} \times 1000}}{{\Delta {T_f} \times {W_A}}}$.

Complete step by step answer:
A liquid freezes at a temperature when its liquid phase vapour pressure is equal to the solid phase vapour pressure. A solution freezes at a temperature when its vapour pressure is equal to the vapour pressure of pure solid solvent. If a non volatile solute is added to the solvent, the vapour pressure decreases and at a lower temperature, it becomes equal to the vapour pressure of solid solvent. The depression in the freezing point $\Delta {T_f}$ is the difference between the freezing point of pure solvent and the freezing point of the solution.

Cryoscopic methods are used to determine the molecular mass of a non volatile solute. In this method, the freezing point of pure solvent is first determined. Then the freezing point of a solution (containing known amount of solute ${W_B}$ in known amount of solvent ${W_A}$) is determined. Then the difference in the freezing point is calculated.

The formula used is ${M_B} = \frac{{{K_f} \times {W_B} \times 1000}}{{\Delta {T_f} \times {W_A}}}$.

Here, ${M_B}$ is the molecular mass of a non volatile solute and ${K_f}$ is the cryoscopic constant , also known as molal depression constant.

Note:
Do not mistake cryoscopic methods with ebullioscopic methods. Both methods are based on similar concepts but the cryoscopic method deals with the depression in the freezing point whereas the ebullioscopic method deals with the elevation in the boiling point.