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?

Answer

`Mw=69.6`

Explanation

Given that
`P^(@)=640 mm Hg`,`P_(S)=600 mm Hg`
`W_(2)=2.175 g`,`W_(1)=39.0 g`
`Mw_(1)=78`
`(P^@-P_S)/P^@=(W_2xxMw_f)/(Mw_2xxW_1)`
`therefore (640-600)/640=(2.175xx78)/(Mw_2xx39)`
`therefore Mw_2=69.6`