[Kingjim9]

Question:
Determine the mole fraction of methanol (CH₃OH) IN THE VAPOR, at 25 degrees C, over a solution composed of only methanol and ethanol (C₂H₅OH). The mole fraction of methanol in the solution is 0.23, and the vapor pressures of pure methanol and ethanol are 155 and 62 torr, respectively, at 25 Degrees C.

My Attempt at a Solution

P_A=X*P_A^°

so...  P_METH=(0.23)*(155) = 35.65 torr

1 Mole_Solution-0.23 = 0.77 Moles of Ethanol

P_ETH=(0.77)*(62) = 47.74 torr


I have no idea what to do from here.

[Borek]

You are very close - what is a total pressure? What is definition of mole fraction? How does number of moles depend on the pressure (for a given V, T)?

[plat_num]

Since pressure and moles are proportional (PV=nRT) wouldn't the mole pressure simply be 155torr/(155torr+62torr)?

[Borek]

No, you don't have pure substances, you have a mixture of them. And you have already calculated their individual pressures.

[plat_num]

No, you don't have pure substances, you have a mixture of them. And you have already calculated their individual pressures.

Could you then do PV=nrt (drop the V since it's constant in both) so P/RT= n? 

Then once you found the moles of both substances in the vapour simply divide each by the total moles present?     

[plat_num]

Wait, I'm obviously missing something...  You would use the 35.65 torr and the 47.74 torr as the pressures to plug into n=P/RT.  That would give you the moles of each pure compound.  Then take THOSE moles, and divide them by the total moles you'd find in the vapour.  Is that right?  or am I way off base here?

[plat_num]

Sorry for being all over the place, I'm all over the place i'm new to this chemistry buisness

Since pressure and moles are proportional I'm thinking you wouldn't NEED to convert anything to moles would you?  All you have to do is add 35.65torr and 47.74torr, and then divide each of your partial pressures by that sum, to get the individual mole fractions of the vapour gas! 

[Borek]

Since pressure and moles are proportional I'm thinking you wouldn't NEED to convert anything to moles would you?  All you have to do is add 35.65torr and 47.74torr, and then divide each of your partial pressures by that sum, to get the individual mole fractions of the vapour gas!

Bingo.

[plat_num]

s#*$, this chemistry stuff is turning out to be pretty cool after all.