Question

: Define boiling point. Write the formula to determine molar mass of a solute using freezing point depression method.

Answer 1

The freezing point depression decreases as the molar mass of the solute increases. Thus, the molar mass of the solute is inversely proportional to the freezing point depression.

Complete step by step answer:
Step 1: Define boiling point.
The temperature at which the vapour pressure of any liquid becomes equal to the atmospheric pressure is known as the boiling point.
At this temperature i.e. at the boiling point, the liquid phase changes to vapour phase.
Step 2: Write the formula to determine the molar mass of solute using freezing point depression method as follows:
The temperature at which the liquid turns into a solid at normal atmospheric pressure is known as the freezing point.
The decrease in the freezing point of a solvent when a non-volatile solute is added to it is known as the depression in the freezing point of the solvent. The formula for the depression in freezing point is,
$\Delta {T_f} = {K_f} \times m$
Where, $\Delta {T_f}$ is the freezing point depression,
${K_f}$ is the freezing point depression constant,
$m$ is the molality of the solution
The molality $\left( m \right)$ of the solution is the ratio of the number of moles of solute to the mass of solvent in kilograms. Thus,
$m = \dfrac{{{\text{Number of moles of solute}}}}{{{\text{Mass of solvent }}}}$
Thus,
${\text{Number of moles of solute}} = m \times {\text{Mass of solvent }}$
The number of moles of a solute is the ratio of mass of solute to the molar mass of solute. Thus,
${\text{Number of moles of solute}} = \dfrac{{{\text{Mass of solute}}}}{{{\text{Molar mass of solute}}}}$
Thus,
${\text{Molar mass of solute}} = \dfrac{{{\text{Mass of solute}}}}{{{\text{Number of moles of solute}}}}$
Thus, the relation between the depression in freezing point and the molar mass of the solute is,
$\Delta {T_f} = {K_f} \times \dfrac{{{\text{Mass of solute}}}}{{{\text{Mass of solvent}}}} \times \dfrac{1}{{{\text{Molar mass of solute}}}}$
Thus, the formula to determine the molar mass of solute using freezing point depression method is,
${\text{Molar mass of solute}} = {K_f} \times \dfrac{{{\text{Mass of solute}}}}{{{\text{Mass of solvent}}}} \times \dfrac{1}{{\Delta {T_f}}}$

Note:
The freezing point depression decreases as the molar mass of the solute increases. Thus, the molar mass of the solute is inversely proportional to the freezing point depression. Thus, an increase in molar mass of solute has a small effect on the freezing point.