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Homework Help: Isobaric Process: Finding Q, E, W.

  1. Dec 16, 2012 #1
    1. The problem statement, all variables and given/known data

    n = 1.46 moles of ideal gas are heated isobarically (at constant pressure) from temperature To = 431 oC to temperature Tf = 1227 oC. Find: change in Energy, Q, W.

    2. Relevant equations

    Change in IE = Q + W
    W (isobaric process) = P(vf-v1)

    3. The attempt at a solution
    change in IE = change in KE (because it is an ideal gas) = (3/2)nRT.
    So, (3/2)(1.46)(8.314 x 10^-3)(1227-431) = 14.5 kJ (no issue here)

    W = P(Vf-Vi), since it is an isobaric process.
    However, how are you supposed to get the volumes of the initial and final state?

    I tried pv = nRT, but am stuck with two unknowns. (Both Pressure, and Volume). Any suggestions? Many thanks in advance !
  2. jcsd
  3. Dec 16, 2012 #2


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    Hint: You don't need separate values for P and V. You only need the combination PΔV.
    Can you relate PΔV to ΔT using the ideal gas law?

    Also, be careful with signs. You wrote ΔIE = Q + W. So, W is work done on the system. Does PΔV represent work done on the system or work done by the system?
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