Question

If $20g$ of a solute was dissolved in $500mL$ of water and osmotic pressure of the solution was found to be $600mm$ of $Hg$ at ${15}^{o}C$, then molecular weight of the solute is:

• A. $1000$
• B. $1200$
• C. $1400$
• D. $1800$

Answer 1

The correct option is B $1200$

Osmotic pressure is a colligative property.

$\pi V=nRT$

$\pi =\frac{n}{V}RT=\frac{mRT}{MV}$

where,
$\pi =$osmotic pressure $=600mm$ of $Hg=600/760atm$
$m=$ mass of solute $=20g$
$M=$ molecular mass of solute $=?$
$R=0.0821Latm{mol}^{-1}{K}^{-1}$
$T={15}^{o}C=15+273=288K$
$V=500mL=0.5L$

Substitute all the above values in (i),

$\frac{600}{760}=\frac{20\times 0.0821\times 288}{M\times 0.5}$

$M=1198\approx 1200$

Molecular mass of solute is $1200$
Hence, the correct option is B.