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Homework Help: Physics question - Ideal Gas Laws

  1. Feb 19, 2017 #1
    Question (see attached diagram):
    PV diagram with 7.5 moles of ideal diatomic gas through cycle a, b and c. What is the highest temperature reached by the gas during the cycle? (multiple choice answers 180, 325, 208 and 100 C, i know answer is 208 C but i'm not getting it!) It is a PV diagram with (triangle), pressure is measured in p(PA x 10^4) and volume is V(m^3) The question is on pg 16 at https://www.scribd.com/doc/287176891:
    Point a => p = 3 p(PA x 10^4) and v = 0.2 V(m^3)
    Point b => p = 5 p(PA x 10^4) and v = 0.6 V(m^3)
    Point c => p = 3 p(PA x 10^4) and v = 0.6 V(m^3)

    Attempt at question:
    Point a:
    p = 3 PA x 10^4 (im keeping this and will use R = 8.31 L.KPA/K.mol I THINK PA x 10^4 = kPA!)
    V = 600 Litres (converted cubic metres to litres times by 1000)
    n = 7,5 (this is given even though its diatomic, it will remain as 7,5 since its moles and not molar mass?
    R = 8.31 L.KPA/K.mol T = ? PV = nRT T = PV/Rn T = 3 x 600/8.31 x 7,5

    Using T = nR/PV:
    T = 1800/62.32 T = 28.8 Kelvin

    If i apply this to all the other points then i get nowhere near the 208 C, so this example above shows that one of my values is wrong (and will be wrong for all other points b and c).

    Many thanks for your help!!!!

    Attached Files:

  2. jcsd
  3. Feb 20, 2017 #2


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    Homework Helper

    No, "kilo" means 1000. 3 x 104 Pa=30 kPa.
    Why do you use liters and kPa-s when the volume is given in m3-s and the pressure in pascals? And R=8.31 m3 Pa/(K mol) .
    And the problem asks the temperature in °C.

  4. Feb 20, 2017 #3
    Thanks for your assistance. I'm doing the best to figure this out, my books does not give me the laws clearly. So your saying 3 x 10^4 Pa=30 kPa, i was using 3 kPA. I converted cubic meters to liters as i assumed that if you work with pascals you needed to convert the cubic meters to liters and if you work with atmospheric pressure (atm) you work with cubic meters. I also assumed that if you use the kilopascal then you must use R=8.31 and if you just use pascals its the other R = 0,0821 value.
  5. Feb 20, 2017 #4


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    Homework Helper

    3 x 10^4 Pa=30 kPa, using 3 kPa instead is wrong.
    R can be used in different units. See
    R=8.31 Pa m3/ (K mol) or R=8.31 kPa L / (K mol) .
  6. Feb 20, 2017 #5

    Ok got it, thanks this helps ten millions times i think i can finish the question now~
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