Question

How do colligative properties affect freezing point?

Answer 1

Hint: The colligative property depends on the concentration of the solute added in the solvent and not on the nature of the solute. The depression in freezing point is one of the colligative properties which is directly proportional to the molality.

Complete step by step answer:
A colligative property is defined as the property of a solution which is dependent on the ratio between the total number of solute present in the solution to the total number of solvent particles. The colligative property is not dependent on the chemical nature of the component’s present in the solution. The depression in freezing point is one of the colligative properties.
The freezing point is defined as the point at which any substance in liquid form starts to freeze also it is defined as the temperature at which the vapour pressure of the liquid becomes equal to the vapour pressure of the solid.
According to Raoult’s law when a non-volatile solute is added to the solvent, the vapour pressure of the solvent decreases by the action of solute at the vapour pressure becomes equal to the solid solvent at a lower temperature. This results in the depression in the freezing point.
The freezing point depression is defined as the lowering of the freezing point of the solvent by the addition of the solutes.
The depression in freezing point is given by the formula as shown below.
$\Delta {T_f} = i \times {K_f} \times m$
Where,
$\Delta {T_f}$ is the depression in freezing point.
i is the Van’t Hoff factor
${K_f}$ is the cryoscopic constant
m is the molality
When, the $T_f^0$ is the freezing point of the pure solvent, ${T_f}$ is the freezing point of the solution and $\Delta {T_f}$ is the depression in freezing point.
So, the freezing point of the solution will be given as
$\Delta {T_f} = T_f^0 - {T_f}$
The depression in freezing point is the colligative property which causes the freezing point of the solution.

Note: The colligative property i.e, freezing point depression can be used to calculate the molecular weight of the component which is dissolved. To complete the calculation one must know the value of mass of solute and solvent and also the freezing point of the pure solvent and the solution.