Question

Define ebullioscopic constant. Write its unit.

Answer 1

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

Complete answer:
Colligative properties are those properties of a solution whose value depends only on the number of the solute particles and not on the properties of the solute.
Elevation in boiling point represents the increase observed in the boiling point of a solution containing a non- volatile solute dissolved in it as compared to the boiling point of the pure solvent. This elevation in boiling point is given by the formula,
$\Delta {T_b} = {K_b} \times m \times i$
Where, ${K_b}$ is the molal elevation constant or the ebullioscopic constant.
$m$ is the molality of the so formed solution.
And, $i$ is the Van’t Hoff Factor.
From this formula, the formula for molal elevation constant can be written as ${K_b} = \dfrac{{\Delta {T_b}}}{m}$
Thus, we can conclude by saying that the ebullioscopic constant or molal elevation constant is the increase in the boiling point of a solution when its molality is equal to unity. In other words it is the increase occurring in the boiling point when one mole of a non- volatile solute is added to $1{\text{ kg}}$of pure volatile solvent. It is a characteristic constant and its value depends only on the nature of solvent, not solute.

Note:
The unit of the ebullioscopic constant is given as ${\text{K kg mo}}{{\text{l}}^{ - 1}}$
The formula for the ebullioscopic constant can be given as, ${K_b} = \dfrac{{RT_b^2}}{{1000{l_v}}}$