Question

At 63.5 degreesC, the vapour pressure of H2O is 23.3kPa and that of ethanol is 53.3kPa. A solution is made by mixing egual masses of H2O and C2H5OH.
a. what is the mole fraction of ethanol in the solution?
b. assuming ideal-solution behaviour, what is the vapour pressure of the solution at 63.5 degreeC.
c. what is the mole fraction of ethanol in the vapour above the solution.

Answer 1

Read this:
http://en.wikipedia.org/wiki/Raoult%27s_law

Answer 2

thenkyou. i missed reading that pert on that page before. i knew it was something simple.
i now have parts A and B but how do you work out C? i have the mole fraction of ethanol from part A. why is it not the same?

Answer 3

The mole fraction of the LIQUID is moles liquid component/total moles liquid.
The mole fraction of the VAPOR is moles vapor component/total vapor moles. Since the vapor pressures are not the same the vapor is more concd in the more volatile component. That's one reason fractional distillation works.

Answer 4

so do i use the moles i would have from part A? surely not, becayse they just give me the mole fraction of the solution.
what do i use? i have the answer of 0.472- don't know how to get it.
i feel so dumb and have my exam on 23rd.