vapor-pressure depression

[marspotter]

Hi. I am going through TPR G. Chem book and I seem to stuck on the concept of vapor-pressure depression and the equation:

∆PA=-XBP°A

I would truly appreciate if anyone could explain this easier.

Thanks!! 🙂

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[engineeredout]

Basically, vapor-pressure depression as I understand it can be explained as follows:

Lets say you have a pure solvent X, which has a vapor pressure of 1000 torr.

Now you add either a non-volatile solute or a solute with a lower vapor pressure. In the case of the non-volatile solute, the vapor pressure of the solution now decreases, because there is less surface area of the solvent exposed so less solvent molecules will exist in the vapor phase.

This will follow raoults law which is P = XP0 where X is the mole fraction of the solvent, and P0 is the vapor pressure of the pure solvent.

If the solute added is volitile, then the equation becomes P = XP0 + (1-X)P1 where P1 is the vapor pressure of the solute (this is for binary solutions, which I believe is all you'll have to worry about on the mcat).

To follow the example, lets say the mole fraction of the solvent is .95. With the non-volatile solute the vapor pressure becomes .95*1000 = 950 torr. If we assume the solute is volatile with a pure vapor pressure of 500, then the pressure becomes P = .95*1000 + .05*500 = 975 torr.

Am I being clear?



As an aside, 1000 torr is a very high vapor pressure. Atmospheric pressure is 760 torr. If the vapor pressure is higher than atmospheric pressure the solution would boil.

 

[marspotter]

Thanks for the fast reply.

However, I still have trouble with your examples as it sort of contradicts what the book says.

This is what the TPR book says,

Raoult's law is classically used to describe the effects of adding a non-volatile solute and does not hold up as well for volatile solutes.

So, with your comparison between the non-volatile solute and volatile one and the equation, I am still confused.

I really appreciate the fast reply, though.
👍

 

[engineeredout]

It is assumed that Raoults law holds true for ideal solutions. The TPR book is correct because in reality, Raoults law only works for high concentrations of the substance (IE the solvent) but not the solute.

For the solute there is a different equation called Henry's law which is P = kX where k is henry's constant which is unique to every substance.

And fast replies are due to the fact that I'm at work, my boss isn't here, and I'm bored. 😴


I just looked on wiki and they provide a good description:

http://en.wikipedia.org/wiki/Henry's_law#Henry.27s_law_versus_Raoult.27s_law

 

[marspotter]

Thanks for the clarification.
🙂