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

Explanation

`(p_(A)^(@) - p_(A))/(p_(A)^(@)) = x_(B) = (w_(B) xx M_(A))/(M_(B) xx w_(A))`
`(2.0 - 1.5)/(2.0) = (4 xx 80)/(M_(B) xx 40)`
`M_(B) = (4 xx 80 xx 2.0)/(0.5 xx 40) = 32 g mol^(-1)`