10% (w/v) solution of urea is isotonic with 6% (w/v) solution of a non-volatile solute X, what is the molecular mass of solute X?

Isotonic solutions have same osmotic pressure at a given temperature.

$\therefore C_{1}RT=C_{2}RT$

$\text{i.e. }{C}_1=C_2$

$\frac{n_1}{V}=\frac{n_2}{V}\Rightarrow n_1=n_2$

No. of moles of urea $=\frac{10}{60}=\frac{1}{6}$

Weight of solute, X = 6 g

No. of moles of solute $x=\frac{6}{M}$

For isotonic solution, $n_1=n_2$

$\text{or }\frac{1}{6}=\frac{6}{M}\text{or }\mathrm{M}=36\mathrm{g}{\mathrm{mol}}^{-1}$