The vapour pressure of pure benzene at a certain temperature is `640 mm` of `Hg`. A non-volatile non-electrolyte solid weighing `2.175 g` added `39.0 g` of benzene. The vapour pressure of the solution is `600 mm` of `Hg`. What is the molecular weight of solid substance?

Options

A. 49.5
B. 59.6
C. 69.6
D. 79.82

Answer

C

Explanation

`(p-p_(s))/(p) = (w//m)/(W//M), (640 - 600)/(640) = (w)/(m) xx (M)/(W)`
`(40)/(640) = (2.175 xx 78)/(m xx 39.08)`
`m = (2.175 xx 78)/(39.08) xx (640)/(40)`
`= 64.458 = 69.60`