Pretty Simple Partial Pressure Equation

[plerrius]

There are two flasks connected by a stopcock. One flask is 2.00 L and is filled with H2 gas at 360 torr. The other is 1.00 L and contains n2 gas at an unknown pressure. When the stopcock is released, the final pressure, at 3.00 L, is 320 torr. How would I go about to find the unknown pressure of N2?

 

I know it's rather simple, but I cannot for the life of me figure this out, no matter how much I read my old notes or my book (Zumdahl/Zumdahl).

 

EDIT: So I tried some different problems and I think I've finally found the solution. I got 240 torr as my answer and am fairly sure it's correct.

Edited August 19, 2010 by plerrius

[cypress]

There are two flasks connected by a stopcock. One flask is 2.00 L and is filled with H2 gas at 360 torr. The other is 1.00 L and contains n2 gas at an unknown pressure. When the stopcock is released, the final pressure, at 3.00 L, is 320 torr. How would I go about to find the unknown pressure of N2?

 

I know it's rather simple, but I cannot for the life of me figure this out, no matter how much I read my old notes or my book (Zumdahl/Zumdahl).

 

Well first of all you need to determine the basis for your estimate. Are you going to use Ideal gas assumptions or adjust for compressibility? Are you going to assume perfect mixing and no H2 to N2 interactions? These two answers will affect the difficulty of the equations but not the process for answering this problem.

 

Next you need to think through the problem and devise a strategy. I will help you with this but it won't help you on a test for me to answer this for you. So you need to give it a try first. finally we need to look up the equations that are based on the strategy and plug and chug.

[plerrius]

Well first of all you need to determine the basis for your estimate. Are you going to use Ideal gas assumptions or adjust for compressibility? Are you going to assume perfect mixing and no H2 to N2 interactions? These two answers will affect the difficulty of the equations but not the process for answering this problem.

 

Next you need to think through the problem and devise a strategy. I will help you with this but it won't help you on a test for me to answer this for you. So you need to give it a try first. finally we need to look up the equations that are based on the strategy and plug and chug.

 

 

 

I ended up using Boyle's Law. Tried it with v1p1=v2p2 and the answer looks correct as everything remains true after checking. I'd spent so much time thinking it would be with the ideal gas law that it kind of threw me off, I guess. Sometimes you just overlook the simplest things.

[cypress]

I ended up using Boyle's Law. Tried it with v1p1=v2p2 and the answer looks correct as everything remains true after checking. I'd spent so much time thinking it would be with the ideal gas law that it kind of threw me off, I guess. Sometimes you just overlook the simplest things.

 

 

Good job, but don't forget that boyle's law in the form you used is predicated on ideal gases. It assumes no compressibility adjustments and no interactions. well done if you believe you are are ok using ideal gas assumptions. Do you know when ideal gas is a good assumption for mixed gases?

[plerrius]

Good job, but don't forget that boyle's law in the form you used is predicated on ideal gases. It assumes no compressibility adjustments and no interactions. well done if you believe you are are ok using ideal gas assumptions. Do you know when ideal gas is a good assumption for mixed gases?

 

Well, nRT has to be constant in Boyle's Law. What I quoted in the first post was the exact question. n obviously didn't change because nothing else was added, R isn't going to change because the units remained the same, and no temperature change was mentioned therefore I should assume it's constant. As for the second question, I'd imagine one should generally assume them unless stated otherwise of if it is implied by the problem.

[cypress]

Yes, that strategy will get you through the homework and test problems with good results, but not in applied science like engineering. Ideal gas assumptins work well when pressures are quite low relative to critical pressure, temperatures are reasonably high compared to boiling point and when the gas molecules are stable, with balanced electron affinities so that they don't interact with other molecules. Nobel gasess and pure elementary gasses are good candidates for ideal gas laws. Complex molecules with asymetrical shapes are not.