Question

The vapour pressure of pure benzene at a certain temperature is 640 mm Hg. A non-volatile non-electrolyte solid weighing 2.175 g is added to 39 g of benzene. The vapour pressure of the solution is 600 mm Hg. What is the molecular weight of the solid substance?

• A. 52.25
• B. 65.25
• C. 40.15
• D. 20.75

Answer 1

The correct option is B 65.25

Given that
Weight of solute = 2.175 g
Weight of solvent = 39 g
Molar mass of solvent = 78 g
Let the molar mass of solute=M
$n=\frac{2.175}{M};N=\frac{39}{78}$
n is mole of solute and N is mole of solvent
$p^0=640\text{mm Hg},p=600\text{mm Hg}$
(M is the mol. wt. of solute)
We know,
$\frac{p^0-p}{p^0}=\frac{n}{n+N}$

$\therefore \frac{640-600}{640}=\frac{2.175/M}{\frac{2.175}{M}+0.5}M=65.25g$
Hence option