Question

$10$% of urea is isotonic with $6$% solution of a non-volatile solute $X$. What is the molecular mass of solute $X$?

• A. $6g$ ${mol}^{-1}$
• B. $60g$ ${mol}^{-1}$
• C. $36g$ ${mol}^{-1}$
• D. $32g$ ${mol}^{-1}$

Answer 1

The correct option is C $36g$ ${mol}^{-1}$

As the solutions are isotonic so they will have same osmotic pressure $\left(\Pi \right)$.

${\Pi }_1={\Pi }_2$

$\frac{n_1}{V_1}=\frac{n_2}{V_1}$ {concentration $=\frac{no.ofmoles}{Vol.\left(L\right)}$}

$10$% urea $=$ $10g$ urea in $100ml$ solution

$6$% $X=$ $6g$ $\times$ in $100ml$ solution

$\frac{10\times 1000}{{Mw}_{\left(urea\right)}\times 100}=\frac{6\times 1000}{{Mw}_X\times 100}$

${Mw}_X=\frac{6\times {Mw}_{\left(urea\right)}}{10}$

$=\frac{6\times 60}{10}$

Mol. wt. of $X=36g/{mol}^{-1}$