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Vapor pressure of Water calculation

  1. Feb 24, 2006 #1
    Ok, I need to calculate the vapor pressure of water.

    All the information I am given is that the temp is 43.0 C, and I know the total pressure of the system is 758.3 torr.

    Is there anyway I can calculate this using the Pwater = Ptotal - Pair equation?
    There is also another equation that I have, but it is quite confusing for me. It is ln P = -(H/R)(1/T) + ln A

    PLEASE HELP!! Thanks guys

    Last edited: Feb 25, 2006
  2. jcsd
  3. Feb 26, 2006 #2


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    Staff: Mentor

    If air is covering the water, then the gas phase contains air and water vapor, therefore P(water vapor) = P(total) - P(air).

    Then what is the pressure of air at 43°C? For interest sake, what is the saturation pressure of water vapor at 43°C?
  4. Feb 27, 2006 #3
    still not gettin it..

    Ok i know that Ptotal = Pair + Pwater..

    but... How do I know what Pair is at 43 C? That is the problem. Is there some standard definition of the pressure of air at a given temperature? I'm not sure... If I had that information it would make this problem a snap.

  5. Feb 27, 2006 #4


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    Staff: Mentor

    I thought I could find it online, but that doesn't seem to be easy.

    I'll look in some thermo books, but that will have to wait until morning.

    The interesting thing is that the total pressure of 758.3 torr is less than standard pressure for 760 torr, which I would expect for 43°C.

    See if this helps - http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/dvlp/rh.rxml
    Last edited: Feb 27, 2006
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