[Dev]

Here's a question which I couldn't figure out,

There's a equimolar (1 mole each) solution to A and B. Both are volatile. One mole of the solution is now converted into vapors. Find pressure over solution.

 

[Arkcon]

OK, lets start with what you know -- have you heard of any formulas that relate what you know (mole) to what you need to know (pressure)?  What else do you need to know?

[Dev]

Hi!

Yeah, we have studied solution state chemistry in class.

I'm familiar with the formula:

VP = XaPao + XbPbo
where Xa = mole fraction of A
Pao = VP of pure A

Also, I'm familiar with Dalton's law of partial pressure.

These two formulas are generally enough to solve any question relating to this.

[Dev]

So, ermm, what now?
Should i re-post this question?

[Arkcon]

That's not what's needed.  The formula you quoted don't look useful for this problem, to me.  Dalton's law of partial pressure may be useful, but I'd rather see an associated formula, and not just the law's name.  As an example, if you're using the Ideal Gas Law formula: PV=nRT, you have some unknown number of moles, call it x, and now you have 1 more mole vaporized, x+1, what happens to P, given that all other variables and constants stay the same?

[Dev]

Okay i guess, YaPt = XaPa0 is the Dalton's law.
Ya = mole fraction of A in vapor form
Pt = total   pressure.

I'm also familiar with PV = nRT and PP = KhX (Henry's law) if that will help.

The main problem here is determining in what ratio are the two liquids vaporised.

[Arkcon]

Yeah, that's part of the trick to this question.  But you've been given no initial ratio of liquids, or any measure of how volatile each is.  You're just told:

One mole of the solution is now converted into vapors.

So you have to know there is one more mole of vapor.

[Dev]

Hi!

The question includes the initial state of the solution.
1 mole of A and 1 mole of B.
Now one mole vaporises.

Do you think this is correct?

Assuming x moles of A vaporises.

then Ya = x Yb = 1-x
Xa = 1-x and Xb = x

then (1-x)(Pao)/(x)(Pbo) = x/(1-x)