Question

Camphor is used as solvent to determine molar mass of non-volatile solute by Rast method because for camphor:
(A) It is readily available.
(B) It is volatile.
(C) Molal depression constant is high.
(D) it is solvent for organic substances.

Answer 1

Hint: You can determine the molar mass of non-volatile solute by using the depression in the freezing point. For this purpose, you can use the following expression in your calculation:
\[{\text{M = }}\dfrac{{ - 1000{\text{ }} \times {\text{ W }} \times {\text{ }}{{\text{K}}_{\text{f}}}}}{{{\text{w }} \times {\text{ }}\Delta {\text{T}}}}\]

Complete step by step answer:
When a non-volatile solute is added to the solvent, the freezing point of solution is less than the freezing point of pure solvent. The difference between the freezing point of the solution and the freezing point of the solvent is called the freezing point depression. The freezing point depression is the colligative property and it depends on the number of solute particles and is independent of the nature of the solute.
Write the expression for the depression in the freezing point:
\[\Delta {\text{T = }}\dfrac{{ - 1000{\text{ }} \times {\text{ W }} \times {\text{ }}{{\text{K}}_{\text{f}}}}}{{{\text{w }} \times {\text{ M}}}}\]
Rearrange the above expression:
\[\Rightarrow {\text{M = }}\dfrac{{ - 1000{\text{ }} \times {\text{ W }} \times {\text{ }}{{\text{K}}_{\text{f}}}}}{{{\text{w }} \times {\text{ }}\Delta {\text{T}}}}\]
Here, M,W are the molar mass and mass of the solute and w is the mass of solvent. \[{{\text{K}}_{\text{f}}}\] is the molal depression in the freezing point constant. \[\Delta {\text{T}}\] is the depression in the freezing point.
When M, w and W are fixed, a higher value of \[{{\text{K}}_{\text{f}}}\] will give a higher value of \[\Delta {\text{T}}\] . A higher value of \[\Delta {\text{T}}\] is easy to measure from the experimental point of view. When the value of \[\Delta {\text{T}}\] is high, there is little room for experimental error. Hence, more accurate value of M can be obtained.
Camphor is used as a solvent to determine molar mass of non-volatile solute by Rast method because for camphor the molal depression constant is high.

Hence, the correct option is the option (C).

Note: The molar mass of non-volatile solute can be determined by several methods which includes depression in the freezing point, elevation is the boiling point, the osmotic pressure etc. Out of these methods, osmotic pressure is most preferred as it is the most accurate method.