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Thermodynamics - Ideal gas expansion

  1. Apr 13, 2014 #1
    1. The problem statement, all variables and given/known data
    2.1E5 J of heat enters an ideal gas as it expands at a constant T = 77°C to four times its initial volume. How many moles of gas are there?

    T=350K, Q=2.1E5 J, Vi=x, Vf=4x


    2. Relevant equations
    ΔU=Q-W
    W=[itex]\int[/itex]pdV
    U=nCvT

    3. The attempt at a solution

    I'm not sure if I'm even on the right track here.
    I have W=[itex]\int[/itex]pdV = nRT[itex]\int[/itex]dv/V = nRTln(Vf-Vi)
    I am stuck here because I need to know n to solve for work but I need to know W to solve for n. I am clearly missing something but I can't figure out what it is.
     
  2. jcsd
  3. Apr 13, 2014 #2

    Andrew Mason

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    You are given Q. What is the relationship between Q and W if the process is isothermal?

    AM
     
  4. Apr 14, 2014 #3
    Yes thank you I realized yesterday that Q=W for an isothermal process. However I was still stuck after that. I feel like I should just use the ideal gas equation but I can't do that since I don't know pressure.
     
  5. Apr 14, 2014 #4

    Andrew Mason

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    Express W in terms of n, T and Vi and Vf (check your equation for W - your integration is not correct). You have values for T, and Vf/Vi. If Q = W, express n in terms of Qm T, Vf/Vi

    AM
     
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