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Thermodynamics problem help again

  1. Dec 12, 2004 #1
    Sorry for the second question in just a day, but my proffessor really didn't cover this material very well. Thanks for any help given :).

    A cylinder contains 4.22 mol of helium gas at a temperature of 314 K. The molar specific heat of helium at constant volume is 12.5 J/K mol. How much heat must be transferred to the gas to increase its temperature to 531 K if it is heated at a constant volume? Answer in units of J.

    Well, after reading the problem, I knew that it was an Isochoric Thermal process due to the comment "heated at a constant volume". Therefore this equation applies:

    Change in Internal Energy = Q - W
    - The work is going to be 0 joules
    - Amended equation: Change in internal Energy = Q

    Q = Cn(Tf - Ti)
    - Solving for that: Q = (12.5)(531 - 314) = 2712.5

    I plugged that in as my answer, and it was wrong. Where did I go wrong? Is the problem even setup right from my perspective?
     
  2. jcsd
  3. Dec 12, 2004 #2

    dextercioby

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    Brilliantly,up until the final calculation.U forgot to multiply by the number of moles of He.Your result should ve been 4.22 times larger.It's awkward,because u stated the formula right. :wink:
     
  4. Dec 12, 2004 #3
    Again, thanks very much.
     
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