Vapor composition from bubble-point temp.

[zaboda42]

Suppose you have a mixture of 30 mol% pentane and 70 mol% hexane at 1 atm and 60^oC.

If the T_bubble-point = 55.1^oC, calculate the composition of vapor of the first bubble as the mixture is heated.

I found p^*_pentane = 1392.95 and p^*_hexane = 484.96 using Antoine's equation with T = 55.1.

Now I suppose I must use Raoult's law as:

y_aP = x_ap^*_a(@55.1^oC)

Using this I get x_pentane = (0.7*760)/1392.96 = 0.382

However, the answer sheet says x_pentane = 0.55.

Can anyone figure where I'm mistaken?

 

[Chestermiller]

You applied the equation incorrectly. y is the mole fraction in the vapor, and x is the mole fraction in the liquid.

y = (0.3)(1392.96)/760=0.550