Question

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

• A. $250gmo{l}^{-1}$
• B. $300gmo{l}^{-1}$
• C. $350gmo{l}^{-1}$
• D. $200gmo{l}^{-1}$

Answer 1

Answer: (B)

$300gmo{l}^{-1}$

$10$ g per $d{m}^3$ of urea is isotonic with 5% solution of a non-volatile solute. Hence, their molar concentrations are, equal to each other.
Molar concentration of urea
$=\frac{10g/d{m}^3}{m_{urea}}=\frac{10g/d{m}^3}{m_{60}}=\frac{1}{6}M$
Molar concentration of 5% non-volatile solute.
$=\frac{50g/d{m}^3}{m(non-volatilesolute)}=\frac{50}{m}M$
$\frac{1}{6}=\frac{50}{m}$
or $m=50\times 6=300gmo{l}^{-1}$