[bingo95]

Hello,

I need help to calculate something.

In an enclosed chamber with the volume 100.9 cm^3, I have added 200 µl (0.2 ml) of a solution containing the chemical X. The concentration of the solution with chemical X is 0.05M and 0.5M (yes I have done this twice). The vapor pressure of chemical X is 400 Pa.

What is the concentration of chemical X in the gas phase of the enclosed chamber?

This is not some random test, this is real and I need your help guys. Thank you very much.

[curiouscat]

Why two numbers?  0.05M and 0.5M??

[bingo95]

Why two numbers?  0.05M and 0.5M??

Because I did this experiment 2 times using different concentrations.

[Borek]

Let's start from the very beginning: what is the concentration definition?

[bingo95]

Let's start from the very beginning: what is the concentration definition?

Do you mean "M"? M is mol/l.

[Borek]

Do you mean "M"? M is mol/l.

No idea what kind of concentration you want to calculate, but if you go for molarity...

It is number of moles per volume. How many moles of the substance, what volume of the chamber?

[AdiDex]

I think you should use Raoult's law to find partial pressure of that chemical X...
Then Use the Dalton's law of partial pressure to get mole fraction of chemical X in the gaseous phase....

[mjc123]

This is a tough one, because if you apply Raoult's Law unthinkingly, you will get the wrong answer. Because the volume of solution is so small compared to that of the gas, that answer would imply the amount of X in the gas phase is more than all the X initially present in solution. You have to realise that as X goes into the gas phase to establish an equilibrium vapour pressure, the concentration of X in solution will decrease, so you can't use the initial concentration in Raoult's Law. There is, however, enough information given to solve the problem.

[bingo95]

This is a tough one, because if you apply Raoult's Law unthinkingly, you will get the wrong answer. Because the volume of solution is so small compared to that of the gas, that answer would imply the amount of X in the gas phase is more than all the X initially present in solution. You have to realise that as X goes into the gas phase to establish an equilibrium vapour pressure, the concentration of X in solution will decrease, so you can't use the initial concentration in Raoult's Law. There is, however, enough information given to solve the problem.

Dear mjc123, thank you for your comments, they are very enlightening! Could you please show me how to solve the problem? I appreciate all help from everyone guys, thank you so much.

[Borek]

Could you please show me how to solve the problem?

It is up to you to solve, we don't do work for others.

[mjc123]

Well, for a start, can you show that you can use Raoult's law (without correction) to give the "simple" answer? Once you can do that, we can think about modifying it.

[bingo95]

Well, for a start, can you show that you can use Raoult's law (without correction) to give the "simple" answer? Once you can do that, we can think about modifying it.

Raoult's law states that mole fraction is proportional with vapor pressure. The mole fraction is for instance 0.5M, then the vapor pressure is already known and 400Pa. There is a very simple equation for Raoult's law that does not take into account diffusion into a set volume. I must admit that based on these clues I can not advance.

[curiouscat]

Well, for a start, can you show that you can use Raoult's law (without correction) to give the "simple" answer? Once you can do that, we can think about modifying it.

Raoult's law states that mole fraction is proportional with vapor pressure. The mole fraction is for instance 0.5M, then the vapor pressure is already known and 400Pa. There is a very simple equation for Raoult's law that does not take into account diffusion into a set volume. I must admit that based on these clues I can not advance.

No diffusion etc. is needed if you only want the equilibrium solution. Those are for studying the transient.

[bingo95]

Well, for a start, can you show that you can use Raoult's law (without correction) to give the "simple" answer? Once you can do that, we can think about modifying it.

Raoult's law states that mole fraction is proportional with vapor pressure. The mole fraction is for instance 0.5M, then the vapor pressure is already known and 400Pa. There is a very simple equation for Raoult's law that does not take into account diffusion into a set volume. I must admit that based on these clues I can not advance.

No diffusion etc. is needed if you only want the equilibrium solution. Those are for studying the transient.

Hello curiouscat, and thank you for your comment. I appreciate your time, but this is a bit out of my field, so I am not able to proceed even with your clue here.

"Equilibrium solution" and "the transient" mean what exactly? Thank you in advance.

[curiouscat]

Equilibrium = what happens when you wait long enough
Transient= How you get there