Question

Assertion: Molal elevation constant depends on the nature of the solvent.
Reason: Molal elevation constant is the elevation in boiling point when $1mole$ of the solute is dissolved in $1kg$ of solvent.
(A) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
(B) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
(C) Assertion is correct but Reason is incorrect.
(D) Both Assertion and Reason are incorrect.

Answer 1

A liquid is said to be boiling only when the vapour pressure of the liquid becomes equal to the external (in most cases the atmospheric) pressure. The temperature at which this phenomenon occurs is known as the boiling point of that particular liquid.

Complete answer:
Elevation in boiling point is associated with the impact of adding non-volatile solutes to solvents. A liquid in its pure state has a fixed escaping tendency, entropy and a surface area exposed to the atmosphere at a particular temperature. The addition of a non-volatile solute to a solvent affects its entropy, reduces its escaping tendency and lowers its vapour pressure.
A liquid solution that has low vapour pressure requires a higher amount of thermal energy to allow its particles to escape as gaseous molecules and as a consequence the boiling point of the solution is increased as compared to the boiling point of liquid in its pure state.
The boiling point elevation constant, which is also known as molal elevation constant or the ebullioscopic constant is a measure of the change in boiling point of a solution which has unit molality i.e. $1mole$ of solute dissolved in $1kg$ of solvent.
The relationship between boiling point change and the concentration of a solution (measured in molality) is given as follows:
$\Delta {T_b}({\text{change in boiling point)}} = {K_b}({\text{molal elevation constant)}} \times m({\text{concentration in molality)}}$
The molal elevation constant depends upon the characteristics of the solvent, because the changes associated in boiling point with a solution of unit concentration will get significantly impacted by the strength of interactions present inside the solvent molecules.
Hence, the correct option is (b) that both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.

Note:
A non-volatile solute in this case refers to a solid solute or a liquid solute that does not have enough vapour pressure of its own. Such a solute would only occupy the surface available for solvent molecules to vaporize and escape but will not add up any vapour pressure to the existing vapour pressure of the solvent.