Correct Answer – A
`p = p_(A) + p_(B)`
`p = p_(A)^(@) x_(A) + p_(B)^(@) x_(B)`
`= 120 xx (2)/(4) + 80 xx (2)/(4)`
`= 60 + 40 = 100` torr
`y_(B) =` mole fraction of B in the vapour phase
`= (p_(B))/(p_(“total”)) = (40)/(100) = (2)/(5)`

a. `Mw_(B) = (chi_(B)xx1000)/((1 – chi_(B))Mw_(A))`
`{:[(chi_(B)= “mole fraction of solute”),(Mw_(A)=”molar mass of solvent”)]:}`
`1= (chi_(B)xx1000)/((1 – chi_(B))xx18)`
`chi_(B) = 0.0176`
`chi_(A) = 1 – 0.0176 = 0.9824`
`P=P^(@)chi_(A) = 760 xx 0.9824 = 746.62`
`DeltaP = P^(@) -P = 760 – 746.62 ~~13.41`

Correct Answer – A
`m=(x_(B)xx1000)/(1-x_(B)m_(A))x_(B)` =mole fraction of solute
`1=(x_(B)xx1000)/((1-x_(B))xx18)m_(A)` =mole mass of solvent
`x_(B)=0.0176`
`x_(A)=1-0.0176=0.9824`
`p=p^(@)x_(A)`
`760xx0.9824=746.62`
`Deltap=P^(@)-p=760-746.62=13.4`