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Homework Help: Specifict Heat capacity of Air.

  1. Jan 5, 2009 #1
    1. The problem statement, all variables and given/known data

    What is the the specific heat capacity at constant volume of Air?, with temperature 300k and pressure = 1 atm.

    2. Relevant equations

    3. The attempt at a solution

    Do I assume that this is a diatomic ideal gas and thus Cv = 5/2 ?

  2. jcsd
  3. Jan 5, 2009 #2


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    Try it and see how close you get to the experimental value ?

    Cp = 1003.5 J kg−1 K−1
  4. Jan 5, 2009 #3
    Ok, well I've assumed air is a diatomic gas and that:

    Cv = 5/2 R = 20.785

    What other equation could I use to work out the precise value of Cv for air?
  5. Jan 6, 2009 #4


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    A few issues with that:

    1) "Specific" implies per unit mass, not per mole.
    2) Air is a combination of gases, not all of which are diatomic.
    3) Air is not a perfectly ideal gas.
    4) Additional heat capacity arises from diatomic bond stretching.

    Depending on how introductory your thermo class is, issues (2), (3), and (4) may be ignored.
  6. Jan 6, 2009 #5


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    Cv = 20.7643 J mol−1 K−1 for air at STP, pretty close to an ideal gas
    You could re-calculate it assuming 1% Argon (the major non-diatomic consituent of air) but to account for the difference between the nitrogen and oxygen is a much more complicated model.
  7. Jan 6, 2009 #6
    Ok thanks. I might as well post the whole question because I can't seem to get the right answer.

    (Q) Find the thermal conductivity of air under the following conditions:

    T = 300 K
    pressure = 1 atm
    density of air = 1.29 kg/m^-3
    coefficient of viscosity is 1.75 x 10-5 Pa.s

    Average speed of air molecules is 466 m/s
    Mean free path of air molecules is 942 x 10^-10 m

    Do I use equation:

    Thermal conductivity = [n<v>λ / 3][Cv/6.02 x 10^23]

    When I do though, I dont get the correct value....
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