Question

The value of $p^0$ for benzene at certain temperature is 640 mm of Hg. The vapour pressure of solution containing 2.5 gm of certain substance in 39.0 gm of benzene is 600 mm of Hg. The molecular mass of A is:

• A. 65.25
• B. 130
• C. 40
• D. 70

Answer 1

The correct option is D 70

According to Raoult's law,
$\frac{\Delta P}{P^S}=\frac{n}{N}$
Where, n = moles of solute and N = moles of solvent.
Let molecular mass of solute be $M_A$.
So, $\frac{\Delta P}{P^0}=\frac{\frac{2.5}{M_A}}{\frac{39}{78}}$
$\Rightarrow \frac{40}{600}=\frac{2.5\times 78}{M_A\times 39}$
$\Rightarrow M_A=75$