Question

A solution containing $10\,g$ per $dm^{ 3}$ of urea (molecular mass = $60\,g\, mol^{-1})$ is isotonic with a $5\%$ solution of a non-volatile solute. The molecular mass of this non-volatile solute is :

• A. $200\, g\, mol^-1$
• B. $300 \,g \,mol^{-1}$
• C. $400 \,g \,mol^{-1}$
• D. $500\, g\, mol^{-1}$

Answer 1

Answer: (B)

For isotonic solution $\pi_{1}=\pi_{2}$

or $C _{1}= C _{2}$ (conc. in mol/lit.)

(Urea solution) (unknown solution)

$\frac{10}{60}=\frac{5 \times 1000}{ m _{ w } \times 100} $

$m _{ w }=300 \,gm\, mol ^{-1}$