3 replies to this topic

[fastjordy378]

Lets say you fill two identical containers with different amounts of pure water, so that there are different amounts of headspace in each container. The containers are open so that the headspace is atmospheric air before sealing them. If you heat them to 150 degrees Celsius for instance, will the container with larger headspace, and therefore larger amount of moles of air in it, have a greater total pressure than the other one? Or will the total pressure be the same for both containers? I realize the vapor pressure of water is only a function of temperature and that it's the same for both containers.

Thanks for the help!

Edited by fastjordy378, 21 June 2011 - 09:38 PM.

[MrShorty]

An interesting question. I don't want to give the answer away, because you need to be able to reason through these problems on your own. Perhaps a nudge:

will the container with larger headspace, and therefore larger amount of moles of air in it, have a greater total pressure than the other one?

I'm assuming you are using the ideal gas law for this analysis. It is true that the P is directly related to n, so one might conclude that the vessel with more air in it has the higher pressure. However, the vessel with more air has a larger headspace (V is larger) and P is indirectly related to V. So, in addition to looking at how P is related to n, one also has to consider how P is related to V. The ideal gas law is a simple expression, so the algebra should be easy enough. I would suggest you walk through each state until you can best see how P is going to change.

[EyMiller245]

You can use the ideal gas (PV = nRT) law to find a qualitative relationship for this particular system.

Note that this system involves water, which should never be treated as an ideal gas for any sort of quantitative analysis.

Edited by EyMiller245, 23 June 2011 - 10:37 AM.

[kkala]

Lets say you fill two identical containers with different amounts of pure water, so that there are different amounts of headspace in each container. The containers are open so that the headspace is atmospheric air before sealing them. If you heat them to 150 degrees Celsius for instance, will the container with larger headspace, and therefore larger amount of moles of air in it, have a greater total pressure than the other one? Or will the total pressure be the same for both containers? I realize the vapor pressure of water is only a function of temperature and that it's the same for both containers.

Attached "conPress.xls" is an example suggesting an answer (subject to comments). Hopefully it helps.

Attached Files

•  conPress.xls   18KB   31 downloads