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Homework Help: Cp/Cv Value for H2O and H2

  1. Jul 14, 2005 #1


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    I need to find out the Cp/Cv value for Water Vapor and Hydrogen gas at 2755 K and a molar ratio of: H2O/H2:48.6112/62.5 I would at least like the Cv and Cp values for water vapor at 2755 K. A correct answer will be greatly appreciated.
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  3. Jul 15, 2005 #2
    from the equation in my thermodynamic textbook this is my equation

    Cp(T) steam = 32.24 + 0.1923e-2*T + 1.055e-5*T^2 - 3.595e-9*T^3

    However, there are a few things i need to mention here

    First the units are kJ/kmol-K so you need to divide by 18 to get kJ/kg-K

    Also, this equation is over the range of 273K - 1800K with a maximum error of 0.53%, so its pretty small, If i use the same equation at 2755 K I get

    Cp(2755) Steam = 42.4393 kJ/kmol-K or

    Cp(2755) Steam = 2.35 kJ/kg-K

    As for the Cv values, my text does not have an equation
  4. Jul 15, 2005 #3


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    I have a computer database that's giving me 1.257
  5. Jul 15, 2005 #4


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    You could use Cp - Cv = R = 8.314 kJ/K-kmol, as a pretty good approximation.

    So, Cv = 42.439 - 8.314 = 34.125 kJ/K-kmol

    Hence, Cp/Cv = 1.244, which is not too far from the number quoted by Q_Goest.
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