Question

The vapour pressure of benzene at a certain temperature is 200 mm Hg. At the same temperature the vapour pressure of a solution containing 2 g of non volatile non electrolyte solid in 78 g of benzene is 195 mm Hg. The molecular weight of solid is:

• A. 20
• B. 40
• C. 60
• D. 80

Answer 1

Answer: (D)

Use Raoults law to find molecular mass of solute.
Given, $p^{o}=$ Vapour pressure of $C_{6} H_{6}$
$=200\, mm \, Hg$
$p=$ vapour pressure of solution $=195 \, mm\, Hg$
$W=$ weight of solvent $\left(C_{6} H_{6}\right)=78 g$
$M=$ molecular wt. of $C_{6} H_{6}=78$
$w=$ weight of solute $=2 g$
$m=$ molecular wt. of solute $=$ ?
$\frac{p^{o}-p}{p^{p}}=\frac{w \times M}{m \times W}$
or $\frac{200-195}{200}=\frac{2 \times 78}{m \times 78}$
or $\frac{5}{200}=\frac{2}{m}$
$\therefore m=80$