Question

Define molal elevation constant or ebullioscopic constant.

Answer 1

Hint: The molal elevation constant, commonly known as the ebullioscopic constant is used in the calculation of the elevation in boiling point of a solution when a non- volatile solute is added to a pure volatile solvent. To answer this question, you must recall the colligative properties, mainly, elevation in boiling point.

Complete answer:
Colligative properties are those properties of a solution which depend only on the number of the solute particles present in the solution.
Elevation in boiling point describes the increase in the boiling point of a solution containing a non- volatile solute dissolved in it than the boiling point of the pure solvent. Addition of the non- volatile solute is responsible for increase in boiling point of the solution as compared to that of the pure solvent. This elevation in the boiling point is represented as,
$\Delta {T_b} = {K_b} \times m \times i$
Where, ${K_b}$ is the ebullioscopic constant or the molal elevation constant.
$m$ is the concentration of the solution, expressed in terms of molality.
And, $i$ is the Van’t Hoff Factor.
From this formula, we can write the molal elevation constant as ${K_b} = \dfrac{{\Delta {T_b}}}{m}$

Note:
Molal elevation constant or the ebullioscopic constant is the elevation in boiling point of a solution when its molality is equal to unity. In other words it is the increase in the boiling point when one mole of a non- volatile solute is added in $1{\text{ kg}}$ of solvent. It is a characteristic constant whose value dependent only on the nature of solvent but not the solute.The formula for the ebullioscopic constant can be given as, ${K_b} = \dfrac{{RT_b^2}}{{1000{l_v}}}$