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Homework Help: Find Heat Energy Needed to Expand Gas to Triple Volume

  1. Mar 20, 2016 #1
    1. The problem statement, all variables and given/known data
    When 0.40 mol of oxygen(O2) gas is heated at constant pressure starting at 0 degrees C, how much energy must be added to the gas as heat to triple its volume? (The molecules rotate but do not oscillate)

    2. Relevant equations
    Value of Cp for Oxygen I used is 0.919 kJ/kg K
    3. The attempt at a solution
    I used the equation pV=nRT to get the value of pV at 273K which was pV=907.5. I tripled that value to get 2722.5. Then, using p1V1/T1=p2V2/T2, I plugged in the values to get the final temperature(819K). From that I used Q=mcdT which gave me the answer of 3211kJ. The answer in the back was 6.4 kJ.
  2. jcsd
  3. Mar 20, 2016 #2
    You must have missed a conversion factor somewhere, your temperatures are correct. For an isobaric process and constant heat capacity:
    [tex]Q= \Delta H = m C_P \Delta T[/tex]
    [tex]Q = 0.4 \ mol \left(0.032 \frac{kg}{mol} \right) \left( 0.919 \frac{kJ}{kg \cdot K} \right) (819.45 \ K - 273.15 \ K) = 6.426 \ kJ[/tex]
  4. Mar 20, 2016 #3
    My mistake was using gmol-1 instead of kgmol-1 for the molar mass. Thanks!!
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