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. 6.96
B. 65.3
C. 63.8
D. none of the above

Answer

B

Explanation

`(P_(1)^(o)-P)/P_(1)^(o)=(W_(2)//M_(2))/(W_(1)//M_(1)+ W_(2)//M_(2))`
`(640-600)/640=(2.175//M_(2))/(2.175//M_(2) + 39//78)`
or `0.0625(2.175/M_(2)+0.5)=2.175/M_(2)`
or `0.9375 xx2.175/M_(2)=0.0625 xx 0.5`
or `M_(2)=65.3`