[evilamoeba]

Consider a solution containing liquids A and B where the mole fraction of B is 0.60. Assuming ideality calculate the mole fraction of B in the vapour at equilibrium wiht this solution at 25 degrees celcius. (The vapour pressures of Pure Liquid A and B @ 25 degrees celcius are 200 torr and 400 torr, respectively).


Please help me i'm stuck! Here's my working so far:


Use Dalton's Law of partial Pressures to get Psoln.

Ptotal = P1 + P2
= 200 + 400
= 600 torr

Raoult's Law:

Psoln = Xsolvent . Posolvent
600 = 0.60 . Posolvent
1000 = Posolvent


? = x . 1000

This is where I have got up to! I'm confused over what the value of "Psoln" should be at equilibrium!


Cheers in advance.

[DevaDevil]

for an ideal gas/liquid goes Raoults Law:

partial pressure of the gas = gas pressure of the pure liquid compound * mole fraction in the solution
p_i = P_i * X_i

this will give you the partial pressures of the compounds in your mixture

then you have mole fraction of B in the gas mixture by partial pressure of B / total pressure of the gases p_B / ( p_B + p_A )

Daltons law is incorrectly used by you, because the 200 and 400 torr are not partial pressures, but pressures of the pure compounds.