Question

Two liquids X and Y form an ideal solution. The mixture has a vapour pressure of 400 mm at 300 K when mixed in the molar ratio 1:1. when mixed in the molar ratio of 1:2 at the same temperatre the vapour pressure of the mixture is 350 mm. The vapour pressure of the two pure liquids X and Y respectively are
A. `250 mm, 550 mm`
B. `350 mm, 450mm`
C. `350mm, 700mm`
D. `550 mm, 250mm`

Answer 1

In 1st case, `n_(X)=n_(Y)`
`therefore x_(X)=0.5` and `x_(Y)=0.5`
`therefore p_(s)=p_(X)^(@)x_(X)+p_(Y)^(@)x_(Y)`
`400=0.5p^(@)Y=800` ….(i)
In 2nd case, `n_(X)//n_(Y)=1//2`
`therefore x_(X)=1//3` and `x_(Y)=2//3`
`therefore 350=(1//3)p^(@)X +(2//3)p^(@)Y`
Or
`p^(@)X + 2p^(@)Y = 1050` ....(ii)
From (i) and (ii)
`p^(@)Y=250` and `p^(@)X=550`