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Ideal gas entropy

  1. Apr 8, 2010 #1
    1. The problem statement, all variables and given/known data

    what is the entropy of 1.0 litre of gasoues n2 at Ta=350 K and Pa=2.0 bar given that So = 191.61 J/(K gmol) at To = 298.15 K and Po = 1 bar?

    2. Relevant equations
    I think : S(at 350) - So = NC_p ln (Ta/To) - NR ln(Pa/po)


    3. The attempt at a solution
    My problem is, C_p is not given and can we use this formula if p is not constant.
     
  2. jcsd
  3. Apr 8, 2010 #2

    Mapes

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    1) Are you sure you don't know [itex]c_P[/itex] for an ideal gas?
    2) To see if you can use this equation even if P is changing, how about starting from [itex]dU=T\,dS-P\,dV[/itex], isolating dS, and integrating?
     
  4. Apr 8, 2010 #3
    C_p = 5/2 R;

    I did the integration and I got the right answer. But the question remains, the specific heat capacity at constant pressure; C_p should be used at constant pressure. Here, the pressure is changing? A qualitative expalnation would suffice. Thanks
     
  5. Apr 8, 2010 #4

    Mapes

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    The pressure isn't changing; there's no process occurring. [itex]c_P[/itex] is just a variable.
     
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