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

According to Raoult's law,
`(p_(0)-p_(s))/(p_(0))=(n)/(n+N)`
Let `m` be the molecular mass of solid substance.
`n=(2.175)/(m),N=(39)/(78)=0.5`
(molecular mass of benzene = 78)
`p_(0)=640` mm , `p_(s)=600` mm
Substituting the values in above equation,
`(640-600)/(640)=((2.175)/(m))/((2.175)/(m)+0.5)=(2.175)/(2.175+0.5m)`
`m=(2.175xx16-2.175)/(0.5)=65.25`.