Question

Solution of two volatile liquid $x$ and $y$ obey Raoult's law. At a certain temperature it is found that when the total pressure above a given solution is $400mm$ of $Hg$, the mole fraction of $x$ in the vapour is $0.45$ and in the liquid is $0.65$. what are the vapour pressure of two pure liquids at the given temperature?

Answer 1

According to Dalton's Law of Partial Pressures, Partial Pressure of each component gas is proportional to their mole fractions in the vapour.

Hence,

Partial Pressure of A $=Pa=400\times 0.45=180$
$Pb=400\times (1-0.45)=220$
Also, Partial Pressure of the gas in the vapour = (Vapour Pressure) × (Mole Fraction of Liquid in solution)

Hence,

Vapour Pressure of A $=Va=\frac{180}{0.65}\approx 277mmofHg$
$Vb=\frac{220}{1-0.65}\approx 629mmofHg$