Question

A 5% solution of cane sugar ($mol.wt=342$) is isotonic with 1% solution of substance $X$. The molecular weight of $X$ is:

• A. $34.2$
• B. $171.2$
• C. $68.4$
• D. $136.8$

Answer 1

The correct option is C $68.4$

For Isotonic solution

$\pi =MRT$

${\pi }_1={\pi }_2$

$\because M_{1}RT=M_{2}RT$

$\therefore M_1=M_2$

$\Rightarrow 5$% solution $\Rightarrow$ 5gm solute in 100 ml solution

$\Rightarrow 1$ % solution $\Rightarrow$ 1 gm solute in 100 ml solution

$So,\frac{5}{342}\times \frac{1000}{100}=\frac{1}{M.\omega t}\times \frac{1000}{100}$

$(M.\omega t{)}_2=\frac{342}{5}$

$=68.4$

Hence correct option is C.