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Homework Help: Ideal Gas Thermo

  1. Feb 22, 2010 #1
    1. The problem statement, all variables and given/known data
    Air is heated from 300 to 350K at constant volume, Find 1Q2.

    2. Relevant equations
    1Q2 = m(u2-u1)+1W2
    u2-u1 = cv(t2-t1)

    3. The attempt at a solution
    The PV diagram of the process is straight up, so the area under the curve is 0, so the work done is zero.
    Cv is .717 from the back of the book

    1Q2 = m*.717(350-300)+0

    How do I find the mass? I don't think I'm doing it right.
  2. jcsd
  3. Feb 22, 2010 #2
    Its a isochoric process.

    you are right about no work done.
    using Q = n Cv delta T,

    what is left is finding how much moles of air is heat.
    if not given, just treat it as a variable. more information required. the mass of gas will tell you how many moles there are.
  4. Feb 22, 2010 #3
    That's what I left it as,

    1Q2 = 50*.717*m

    Would that be right?
  5. Feb 22, 2010 #4
    moles = mass / (molecular weight x avogadro's constant)
    avogadro constant = 6.02 x 10^23

    molecular weight is something that you can find on the data booklet.

    you cant leave the mass just like tt.
    the m you wrote is moles not mass.

    may i ask you wad you actually mean by 1Q2. delta heat energy input or delta internal energy.
    delta means change in.
    Last edited: Feb 22, 2010
  6. Feb 22, 2010 #5
    Final heat, and in our thermodynamics class, we don't use moles (n), we use m for mass.
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