Question

A solution of urea contains 8.6 gm/litre (mol. Wt. 60.0). It is isotonic with a 5% (W/W) solution of a non-volatile solute having density 1 g/cc. The molecular weight of the solute will be

• A. 348.8
• B. 34.88
• C. 3488
• D. 861.2

Answer 1

The correct option is A 348.8

Isotonic solutions have same osmotic pressure
As Osmotic pressure = C R T
$\Rightarrow C_1=C_2$
$\Rightarrow \frac{n_1}{V_1}=\frac{n_2}{V_2}$
$5\%$ W/W solution means 100g of solution contains 5 g of solute
$\therefore \frac{\frac{8.6}{60}}{1}=\frac{\frac{5}{M}}{\frac{100}{1000}}$
$\Rightarrow M=\frac{5\times 60\times 10}{8.6}=348.8$