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Questions on pressure and non-ideal gases

  1. May 8, 2013 #1

    I have two questions.

    First, how do I calculate the pressure of a non-ideal gas if I know volume, temperature and the number of moles? The gas in question is water vapor. I suspect a constant for the specific gas will be involved. Which constant is this?

    And secondly, how do I calculate how much energy is stored in a pressurized gas if I know the mass (or the number of moles) and the pressure of the gas?

    Thanks in advance! :smile:
  2. jcsd
  3. May 8, 2013 #2


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    Staff: Mentor

  4. May 8, 2013 #3
    Thanks for your post, good explanation! The problem is that I don't seem to be getting the right answer.

    Water vapor has a density of 0.6 kg/m3 at 100 degree celsius (373 degree kelvin) and standard atmospheric pressure.
    Source: http://www.wolframalpha.com/input/?i=Water+vapor+density+at+atmospheric+pressure

    So if I put in all the values for water vapor except pressure, I should get 1 atm as the answer, but instead I get approximately 0.032 atm.

    These are the values I used:

    P = Unknown
    V = 1 Liter
    a = 5.464 According to the website you linked
    n = 0.6/1000/18 = 3.33*10^-5 moles
    b = 0.0305 According to the website you linked
    R = 0.08260 According to the website you linked
    T = 373 Degree kelvin

    Done the calculation several times, don't understand whats wrong...

    Thankful for help!
  5. May 8, 2013 #4
    It's difficult to say what is wrong with your calculation if you don't show it.

    But the corrections to the ideal gas equation for the given parameters are really small.
    The ideal gas gives approximately 1 atm and so should the van der Waals equation.
    There is an error in the equation on this site, I believe. It should not be P squared, it does not make sense (dimensionally).
  6. May 9, 2013 #5
    Using an equation of state for a substance with hydrogen bonding, especially water, is never going to give you a good answer. Regardless, I looked up the specific volume of saturated water vapor at 1atm and 373.15K using the NIST steam tables, which was .0332 mol/L and used the ideal gas law to find pressure: .082057*373.15*.0332=P=1.0166 atm. You could use the virial equation or compressibility factor, but why bother. Water has tabulated values, make use of em! http://webbook.nist.gov/chemistry/fluid/
    Last edited: May 9, 2013
  7. May 9, 2013 #6
    Thanks for the help everyone, got it right now! :)
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