Question

Boiling point of a $2\%$ aqueous solution of a non-volatile solute $A$ is equal to the boiling point of 8\% aqueous solution of a nonvolatile solute B. The relation between molecular weights of $A$ and $B$ is.

• A. $M_A=4M_B$
• B. $M_B=4M_A$
• C. $M_A=8M_B$
• D. $M_B=8M_A$

Answer 1

Answer: (B)

For $A:100gm$ solution $\to 2gm$ solute A
$\therefore \text{ Molality }=\frac{2/M_A}{0.098}$
For B : $100gm$ solution $\to 8$ gm solute B
$\therefore \text{ Molality }=\frac{8/M_B}{0.092}$
$\because {\left(\Delta T_B\right)}_A={\left(\Delta T_B\right)}_B$
$\therefore \text{ Molality of }A=\text{ Molality of }B$
$\therefore \frac{2}{0.098M_A}=\frac{8}{0.092M_B}$
$\frac{2}{98}\times \frac{92}{8}=\frac{M_A}{M_B}$
$\frac{1}{4.261}=\frac{M_A}{M_B}$
$\therefore M_B=4.261\times M_A$