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:

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Solution

The correct option is D 80
Given $p^{o} = 200 m m H g$
$p = 195 m m H g$
$w =$ weight of solute = 2g
$W =$ weight of solvent = 78 g
$M =$ Molecular weight ofsolvent (benzene)=78
$m =$ Molecular weight solute = ?
$\frac{p^{o} - p}{p^{o}} = \frac{w \times M}{W \times m}$ or $\frac{200 - 195}{200} = \frac{2 \times 78}{78 \times m}$
or $\frac{5}{200} = \frac{2}{m} ∴ m = 80$

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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:

The correct option is D 80Given po=200mmHgp=195mmHgw= weight of solute = 2gW= weight of solvent = 78 gM= Molecular weight ofsolvent (benzene)=78m= Molecular weight solute = ?po−ppo=w×MW×m or 200−195200=2×7878×mor 5200=2m∴m=80