[vi.neon]

Hello everyone! This question is for a general chemistry course.
I thought that I understood what partial pressure and vapour pressure are but this question changed my mind:

"at a certain temperature, a given mixture contains two liquids AB; the volatility of liquid A is twice the volatility of liquid B"

and I had to choose between several options, including:
1) "vapour pressure of A is twice the vapour pressure of B",
2) "in vapour phase, the number of particles of A are twice the particles of B".

Now, I have in mind these definitions:
- partial pressure: given a mixture of gases in a container, it is the pressure one gas would exert if it was alone in that container
- vapour pressure: given a liquid mixture, it is the pressure a component exerts on the surface of the liquid phase once the component reached dynamic equilibrium

I know that the meaning of "pressure" thus its definition is the result of the particles of a gas in a container hitting the walls of the container, therefore the more particles there are the higher the pressure will be: so why should I exclude the 2nd option I mentioned? Seen this doubt I think I don't really figure out the difference between vapour pressure and partial pressure: we can talk about the 1st one when we have a mixture of liquids and we relate to the 2nd one when we only have a mixture of gas, but how is it different to talk about the pressure exerted on the surface of a liquid rather than the pressure exerted on the walls of a container? For a vapour in equilibrium with its liquid phase, could we talk about both vapour pressure and partial pressure being them two different things in the context?

In conclusion...Then volatility is just a synonym for vapour pressure? Reading through my notes I can sort of deduce it but I'm not so sure.

I hope I was able to explain myself, I would appreciate any attempt of answer or examples, thanks in advance!

[AWK]

For real gases, is doubling the number of molecules the same as doubling the vapor pressure?

[vi.neon]

For real gases, is doubling the number of molecules the same as doubling the vapor pressure?

Thanks for replying. The question didn't specify whether we are in a real or ideal condition, but as in the course I asked for we talked about real gases just a little and mainly treated ideal gases (it's a really basic course) I supposed, maybe wrongly, that gases were ideal. Anyway no, because neither interactions between particles nor their volume in real gases are negligible so this affects pressure differently. But I can't still get to the point, as now I'm referring to vapour pressure as it was precisely the gas pressure without any distinction.

[mjc123]

I think "volatility" is one of those words that expresses a general concept (liability to evaporate) but is not a precisely defined technical term. For example for an ideal mixture of A and B, the partial pressure of A vapour is x_AP°_A, where x_A is the mole fraction of A in the liquid and P°_A is the vapour pressure of pure A. The total vapour pressure of the mixture is given by
P = x_AP°_A + x_BP°_B
Now I think that saying the volatility of A is twice that of B is ambiguous, because "volatility" is not precisely defined. It could mean
P°_A = 2P°_B
or it could mean
x_AP°_A = 2x_BP°_B
The latter corresponds to your option 2; option 1 is ambiguous - is it referring to vapour pressures of pure A and B, or of A and B in the mixture?

[vi.neon]

I think "volatility" is one of those words that expresses a general concept (liability to evaporate) but is not a precisely defined technical term. For example for an ideal mixture of A and B, the partial pressure of A vapour is x_AP°_A, where x_A is the mole fraction of A in the liquid and P°_A is the vapour pressure of pure A. The total vapour pressure of the mixture is given by
P = x_AP°_A + x_BP°_B
Now I think that saying the volatility of A is twice that of B is ambiguous, because "volatility" is not precisely defined. It could mean
P°_A = 2P°_B
or it could mean
x_AP°_A = 2x_BP°_B
The latter corresponds to your option 2; option 1 is ambiguous - is it referring to vapour pressures of pure A and B, or of A and B in the mixture?

I am getting a bit confused about this, well I'm sure that if we have the partial pressure of a component doubled (we  can also refer to partial pressure by saying vapour pressure of the component in the mixture, because we're talking about a liquid mixture), then we have the double of the particles of that component as the pressure only depends on the number of particles when the solution is ideal. In conclusion, so far I only found out this about volatility:

"Volatility itself has no defined numerical value, but it is often described using vapor pressures or boiling points (for liquids)",

while in my notes there's written "if P°A = p°B, then A and B have the same volatility". Considering that the right answer turned out to be the first option, I guess my teacher had the pure component vapour pressure as a definition of volatility.

Anyway, maybe I can't understand what the vapour pressure of the pure component is, I'd say it's the pressure the component would exert if that liquid was alone in the same container, but this way it looks quite the same as the definition of partial pressure.. Where is the mistake? Thanks for your time.