# Using Raoult's law and Dalton's law

1. Jul 8, 2006

### softie

Hi there!

I'm having a little problem here and hopefully someone here can help.
I am studying for a Physical Chemistry exam and there is an exercise here I just can't solve. It asks to find the ideal and the real parcial pressure of A, given xA, yA and pt ---- xA being the molar fraction of A in the liquid phase and yA the molar fraction of A in the gaseous phase.
I was thinking of using Raoult's law and Dalton's law, but don't they both give the ideal values of the partial pressure?? How can I get the real values?

Thanks! :)
sara

2. Jul 8, 2006

### GCT

Unless the question asks for a better approximation, it's referring to activity calculations.

3. Jul 22, 2006

### softie

thank you!

yes, that was my doubt, but it's not how it was supposed to be answered. the raoult law is supposed to give us the real partial pressure, and the dalton law the ideal one, because we are considering NPT conditions.

thanks anyway!

p.s.: I did 18.2 / 20 on that exam. :)

4. Jul 22, 2006

### Gokul43201

Staff Emeritus
No, that's not entirely correct. However, I can't tell you what to do with the question unless you post the original question EXACTLY as it appears in the exercise (and it doesn't hurt to include the book and chapter where this problem comes from). Your paraphrasing of it makes it very ambiguous.

But I can say this : Raoult's Law applies almost only to ideal solutions. For real solutions that deviate from ideality, you can still use Raoult's Law for the dominant component (ie: solvent) of a dilute solution. But for the solute in a non-ideal solution, you must use Henry's Law (and replace the vapor pressure of pure A with the Henry's Law constant).