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Homework Help: The final temperature of a gas?

  1. Oct 15, 2011 #1
    1. The problem statement, all variables and given/known data
    Starting with 2.50 mol of N2 gas (assumed to be ideal) in a cylinder at 1.00 atm and 20C, a chemist first heats the gas at constant volume, adding 1.52x10^4J of heat, then continues heating and allows the gas to expand at constant pressure to twice its original volume. Calculate the final temperature of the gas.

    2. Relevant equations
    PV=nRT
    ΔU=Q-W
    ΔU=nCvΔT

    3. The attempt at a solution
    The first part suggests that the system is in a constant volume. Thus:
    ΔU=Q
    nCvΔT=Q
    (2.5mol)(5/2)(R)(Tf-20)=1.52x10^4

    I solve for Tf and I get 312.5C.

    Now the next part suggests the system is at a constant pressure and the volume doubles.
    So I use PV=nRT where n, R and P are constant.
    V/T=V/T
    1/312.5C=2/T

    I solve for and I get 625C. However the answer in the back is 899C. What am I doing wrong?

    Thanks!
     
  2. jcsd
  3. Oct 16, 2011 #2

    Redbelly98

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    Staff Emeritus
    Science Advisor
    Homework Helper

    For this equation, T must be expressed as an absolute temperature, not in degrees C.
     
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