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Homework Help: Monatomic Gas

  1. Dec 28, 2006 #1


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    I really have no Idea where to start... Please help

    1. The problem statement, all variables and given/known data
    A monatomic gas is cooled by 50 C at constant volume by removing 830 J of energy. How many moles of the gas is in the sample.

    2. Relevant equations

    ???Q=mc (delta)T

    3. The attempt at a solution

    I am not sure where to start...a monatomic gas could be any of the noble gases, right? each with a different molecular weight and each would have a different specific heat at a constant volume(c). Is this even the right equation to start with. Thanks.
  2. jcsd
  3. Dec 28, 2006 #2
    The main principle in this question is the First law of Thermodynamics
    By 1st Law, dQ=dW+dU (using d for delta,nevertheless)
    dW=0 as volume is constant
    So, dQ=dU

    Now, for monoatomic gases, Molar specific heat at constant volume is given by 3/2.R,where R=gas const.
    as dU=nCdT, the values can be substituted to get the only unknown n, which is the no.of moles.

    (Monoatomic is explicitly mentioned so that the Cv of the gas can be found out based on equipartition of energy)
  4. Dec 29, 2006 #3


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    Thank you... That was perfect. It is frustrating because the formulas you used were never taught to me. They are in the book but they are outside the material we were required to learn. But my teacher is like that... Any how thanks for your help.
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