The vapour pressure of benzene at a certain temperature is `640 mm Hg`. A non-volatile and non-electrolyte soild weighing `2.175 g` is added to `39.08 g` of benzene. If the vapour pressure of the solution is `6 mmHg`. What is the molecular mass of solid substance?

Options

A. `49.50`
B. `59.60`
C. `69.40`
D. `79.82`

Answer

C

Explanation

According to Raoult's law,
`(p^(@)-p)/(p^(@))=(wxxM)/(mxxW)`
`(640-600)/(640)=(2.175xx78)/(mxx39.08)("M for" C_(6)H_(6)=78)`
`m=(2.175xx78xx640)/(40xx39.08)`
`=69.45~~69.4`