Question

The solution containing $6g$ of an organic compound per litre of the solution shows an osmotic pressure of $1.2atm$ at ${27}^{\circ }C$. Calculate the molecular mass of the compound

• A. $76.4gmo{l}^{-1}$
• B. $188.7gmo{l}^{-1}$
• C. $46.3gmo{l}^{-1}$
• D. $123.2gmo{l}^{-1}$

Answer 1

The correct option is D $123.2gmo{l}^{-1}$

Osmotic pressure:

$\pi =\frac{n}{V}\times RT$

Here n is number of moles

$n=\frac{\text{Given mass}}{\text{Molar mass}}=\frac{w_B}{m_B}=\frac{6}{m_B}$

$V=1LR=0.0821Latmmo{l}^{-1}{K}^{-1}T={27}^{\circ }C=300K$

$\Rightarrow \pi V=\frac{w_B}{m_B}\times RT{m}_B=\frac{w_B\times R\times T}{\pi \times V}$

$m_B=\frac{6g\times 0.0821Latmmo{l}^{-1}{K}^{-1}\times 300K}{1.2atm\times 1L}{m}_B=123.15gmo{l}^{-1}$