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:

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

Tardigrade Admin · Accepted Answer

Use Raoults law to find molecular mass of solute.
Given,

p^{o} =

Vapour pressure of

C_{6} H_{6}

= 200 mm H g

p =

vapour pressure of solution

= 195 mm H g

W =

weight of solvent

(C_{6} H_{6}) = 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}

∴ m = 80

Q. 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:

20

40

60

80