The vapour pressure of pure benzene is `639.7 mm Hg` and the vapour pressure of solution of a solute in benzene at the temperature is `631.9 mm Hg`. Calculate the molality of the solution.

Options

A. 65.25
B. 130
C. 40
D. 80

Answer

D

Explanation

`(P^@-P)/(P^@) = n_2/(n_1 + n_2)`
`(640 = 600)/(640) = (2.5 //m)/(39//78)`
`m= (640 xx 78 xx 2.5)/(39 xx 40) = 80`