Question

What is molal elevation constant? What are its units?

Answer 1

To solve this question, we must first understand the concept of Molal Elevation Constant. Then we need to assess the main points and by considering the molal concentration of solute we can derive its equation and then after we can conclude its SI Unit and in this way we can write the correct answer.

Complete step by step answer:
Molal Elevation: is defined as the elevation in boiling point produced when one mole of solute is dissolved in $1{\text{ }}kg$ i.e. $1000{\text{ }}g$ of the solvent. This is also known as the ebullioscopic constant.
It is represented by ${K_b}$ .
Derivation:Let us assume that the molal concentration of the solute is $m$ .
If we talk about the experiment, then it is being shown that for dilute solutions, the elevation of the boiling point is directly proportional to the molal concentration of solute in the solution.
So we can write, $\Delta {T_b} \propto \,\,m$ ; where m is the number of moles of solute dissolved in $1$ kg of solvent which is also known as molality.
$\Delta {T_b} \propto \,\,m$
$\Rightarrow \Delta {T_b}\,\, = \,\,{K_b}m$ ; where ${K_b}$ represents molal elevation constant.
SI Unit:
Clearly we can conclude that the SI unit of ${K_b}$ is $K\,kg\,\,mo{l^{ - 1}}$ .
So finally, we can conclude that molal elevation constant is related to Elevation in boiling point as $\Delta {T_b}\,\, = \,\,{K_b}m$ and its SI unit is $K\,kg\,\,mo{l^{ - 1}}$ .

Note: Boiling-point elevation describes the phenomenon that the boiling point of a liquid (a solvent) will be higher when another compound is added, meaning that a solution has a higher boiling point than a pure solvent. This happens whenever a non-volatile solute, such as a salt, is added to a pure solvent, such as water. The boiling point can be measured accurately using an ebullioscope.