# Vapor pressure + air pressure?

## Main Question or Discussion Point

Hi, could someone please tell me if the vapor pressure should be combined with the air pressure in a sealed vessel to give the total pressure? For example, if I put room temperature water in a test tube (at sea level) with some head space for air and then seal it, the pressure inside would be 1 bar, would it not? If I then heated the water inside to 100C, the vapor pressure should equal 1 bar also, shouldn't it? Does that mean that the air pressure and the vapor pressure combine to give a total pressure of around 2 bar? Or is the total pressure closer to 1 bar because the air pressure decreases the amount of water that can vaporize at the given temperature? I apologize for my somewhat rudimentary questions and many thanks for your help

James

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Thanks, UltrafastPed. So according to Dalton's Law, the partial pressure of the air at 1 bar and the partial pressure of the water vapor at 100C, combine to give a total pressure of 2 bar?

If "Dalton's law is not exactly followed by real gases" and "those deviations are considerably large at high pressures", would 2 bar be considered high enough (for air + water) to cause much of a deviation? Or will it still be relatively close to 2 bar? Thanks again for your help.

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UltrafastPED
Gold Member
If those are the correct partial pressures, then yes.

Chestermiller
Mentor
Hi, could someone please tell me if the vapor pressure should be combined with the air pressure in a sealed vessel to give the total pressure? For example, if I put room temperature water in a test tube (at sea level) with some head space for air and then seal it, the pressure inside would be 1 bar, would it not?
Yes. Also, as you are in the process of sealing the test tube, the water vapor partial pressure in the head space would attain the equilibrium vapor pressure of water at the test tube temperature, so that the partial pressure of the air in the head space would be slightly less than 1 bar.
If I then heated the water inside to 100C, the vapor pressure should equal 1 bar also, shouldn't it?
Yes.
Does that mean that the air pressure and the vapor pressure combine to give a total pressure of around 2 bar?
Roughly yes. Of course, the air partial pressure would also increase when you heat the test tube, so the total pressure would be greater than 2 bar.
Or is the total pressure closer to 1 bar because the air pressure decreases the amount of water that can vaporize at the given temperature?
No