Question

A solution containing $8.6g{L}^{-1}$ of urea is isotonic with a $5\%$ solution of unknown solute. The molar mass of the solute will be

• A. $348.9gmo{l}^{-1}$
• B. $174.5gmo{l}^{-1}$
• C. $87.3gmo{l}^{-1}$
• D. $34.89gmo{l}^{-1}$

Answer 1

Answer: (A)

Urea $\to 8.6g/L=$ i.e., $\left(\frac{w}{\text{ volume }}\right)$
Molar mass $=N{H}_{2}CON{H}_2$
$\Rightarrow 14\times 2+4+12+16$
$\Rightarrow 60$
isotonic $\to \left[{\pi }_1={\pi }_2\right]$
$C_{1}ST=C_{2}ST$
$C_1=C_2$
$\frac{W}{M_w\times V}=\frac{W}{M_w\times V}$
$\frac{8.6}{60}\Rightarrow \frac{5\times 1000}{M_w\times 100}$
$M_w=\frac{5\times 60\times 10}{8.6}$
$=348.8g/mol$
Errata : $5\%$ solution of unknown solute but it must be written $5\%$ weight/volume means $5g$ of solute present in $100mL$ of solution