Question

Elevation in boiling point was ${0.52^o}C$ when $6g$ of compound X was dissolved in $100g$ of water. Molecular mass of the compound M in $gmo{l^{ - 1}}$ is: (${K_b}$ for water is $0.52K{m^{ - 1}}$.)
A.$120$
B.$60$
C.$600$
D.$180$

Answer 1

We need to know that the elevation in boiling point explains the method that the boiling point of a liquid will be more by adding another compound. The molecular mass of the compound can be found by using the equation of elevation of boiling point. The amount of solute is directly proportional to the amount of change in boiling point. The boiling point of solution is always higher than that of the solvent and it takes place by adding non – volatile solute like salt to the pure solvent like water.

Complete answer:
As we know that the molecular mass of the compound is not equal to $120$. Hence, option (A) is incorrect.
The molecular mass of the compound can be find out by using the equation of elevation of boiling point and that is,
$\Delta {T_b} = {K_b}m$
Where, ${T_b}$ is the boiling point temperature and ${K_b}$ is equal to the proportionality constant and m is equal to the molar mass of the solvent and the equation is also written as,
$\Delta {T_b} = {K_b} \times \dfrac{{\text{Mass Of Solute}}}{{\text{Molar Mass Of Solute} \times \text{Mass Of Solvent}\left( {ing} \right)}} \times 1000$
Mass of solute is equal to $6g$and mass of solvent is equal to $100g$. Substituting the given values in the above equation will get the molecular mass of the compound.
$0.52 = 0.52 \times \dfrac{6}{{M \times 100}} \times 1000$
By rearranging the equation,
$M = 60gmo{l^{ - 1}}$
Therefore, the molecular mass of the compound is equal to $60gmo{l^{ - 1}}$.
The molecular mass of the compound is not equal to $600$. Hence, option (C) is incorrect.
The molecular mass of the compound is not equal to $180$. Hence, option (D) is incorrect.

Hence, option (B) is correct.

Note:
We need to remember that the molecular mass of the compound is found by using the equation of elevation of boiling point. The boiling point elevation represents the increase in the boiling point of a solvent by adding a solute. The vapour pressure of the solvent will be less by adding the solute and it takes place due to the displacement of the molecule of solvent by the solute.