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Topic: Vapor Pressure  (Read 7440 times)

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Offline BetaAmyloid

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Vapor Pressure
« on: June 30, 2011, 04:28:21 PM »
So, we did this experiment where we placed a graduated cylinder containing a small amount of air upside down into a beaker. We watched the bubble increase as temperature increased and vice versa.

When doing calculations, we used a low temperature along with the volume of air at that temperature to calculate nair, assuming that the vapor pressure of water was negligible at this temperature.

However, there was a question regarding vapor pressure: What would happen to the number of moles of air (observing volume) if the vapor pressure was not assumed negligible under 5oC, and the real vapor pressure was taken into affect?

I assumed that since vapor pressure is not assumed negligible, therefore increased, that the pressure of the water on the air within the graduated cylinder would cause the air volume to decrease, therefore "decreasing" the moles of air. However, apparently the vapor pressure taken into account increases moles of air.

How is this so?

Thanks in advance.
Discovery consists of seeing what everybody has seen and thinking what nobody has thought - Albert Szent-Györgyi

Offline BetaAmyloid

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Re: Vapor Pressure
« Reply #1 on: July 03, 2011, 01:05:05 PM »
Maybe I can clarify to help get an answer!

I assumed that since the water vapor pressure was not negligible, therefore not equal to atmospheric pressure, then the atmospheric pressure would be "pressing" down on the water since water, in the liquid phase, has less pressure than atmospheric pressure. So if this was true, atmospheric pressure would be "pressing" on the water, making the water "press" on the air, causing the air to "decrease" in its number of molecules.

However, the answer was that if water vapor pressure was not assumed negligible, then the number of air molecules would actually "increase." How is this possible if there is more force pressing on the air?

Thanks.
Discovery consists of seeing what everybody has seen and thinking what nobody has thought - Albert Szent-Györgyi

Offline Yggdrasil

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Re: Vapor Pressure
« Reply #2 on: July 03, 2011, 01:24:37 PM »
Vapor pressure matters here because it changes the number of molecules in the air depending on how much water vapor there is.  As you cool the air, it takes up less volume because the molecules are not moving around as vigorously.  However, as you cool down the air, you also decrease the vapor pressure of the water.  More water molecules will return to the liquid phase and the total number of molecules in the gas phase will decrease.  Thus, you will overestimate the magnitude of the volume change due to simply cooling the gas.

Offline BetaAmyloid

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Re: Vapor Pressure
« Reply #3 on: July 04, 2011, 12:10:41 AM »
Thanks Yggdrasil! But what I was asking wasn't how vapor pressure changes due to temperature, but how the number of gas particles of air change as the vapor pressure of water decreases. Here is the exact question: "You have assumed the vapor pressure of water below 5 °C to be negligible. How would the inclusion of its actual vapor pressure affect your results?"

Here, I'll try to show my thinking process through 2 diagrams.
-------------
Diagram assuming vapor pressure of water is negligible:

Here, the water vapor pressure is equal to the atmospheric pressure, therefore having no affect on the air volume.
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Diagram assuming vapor pressure of water is less than atmospheric pressure:
[URL=http://imageshack.us/photo/my-images/51/35192201.png/]

Here, the water vapor pressure is less than the atmospheric pressure, therefore the atmospheric pressure is pressing down on the water which is then pressing up on the air volume - decreasing the volume of air.
-------------

This was my thinking, but the answer is supposedly "increase the air volume." How can this be possible?

Thanks.
Discovery consists of seeing what everybody has seen and thinking what nobody has thought - Albert Szent-Györgyi

Offline Yggdrasil

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Re: Vapor Pressure
« Reply #4 on: July 04, 2011, 12:56:48 PM »
I think you may be misunderstanding the concept of vapor pressure.  Vapor pressure is merely the measure of the amount of water present in the gas phase at equilibrium.  Because this vapor increases the number of moles of gas, it will increase the total pressure pushing down on the liquid (both at the top of the beaker and inside of the cylinder).  It is not a force, as your diagrams seem to indicate, pushing up against the atmospheric pressure.

Vapor pressure does not have to be equal to atmospheric pressure, and in most cases it is well below atmospheric pressure (the vapor pressure of water is equal to the atmospheric pressure only at the boiling point of water).  The case where vapor pressure is negligible is the case where you assume there is no water vapor in the air.

Offline BetaAmyloid

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Re: Vapor Pressure
« Reply #5 on: July 04, 2011, 07:36:56 PM »
That helps! I guess I had a misunderstanding of vapor pressure!

I get it now, I appreciate it! :)
Discovery consists of seeing what everybody has seen and thinking what nobody has thought - Albert Szent-Györgyi

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