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Homework Help: Liquifying Nitrogen

  1. Aug 15, 2014 #1
    1. The problem statement, all variables and given/known data

    A rigid cylinder contains 9.0g of nitrogen at 20 ∘C.

    What is the minimum amount of heat energy that must be removed to liquify the nitrogen?

    2. Relevant equations

    Heat Energy
    H = Cp * m * ΔT

    H = Heat Energy
    Cp = Heat Capacity
    m = mass (in Kg)
    ΔT = change in temperature (in Celsius)

    Heat of Condensation (Equal to Vaporisation)

    H = Hc * m

    Hc= Heat of condensation
    m= mass (in moles)

    3. The attempt at a solution

    Heat Capacity of nitrogen = 1.04 kJ/kg*K.
    Nitrogen is a liquid at -196 C, so ΔT = -196 - 20 = -216

    H = Heat Capacity * mass * ΔT

    H = 1.04 kJ/kg*K * 0.009 kg * 215 = 2.02 kJ


    H= Heat of condensation * mass (in moles)

    H= 5.56 kJ*mol * 0.32 moles = 1.78 kJ

    2.02 kJ + 1.78 kJ = 3.8 kJ

    3.80 kJ of heat energy is required to liquify the nitrogen.

    Answer isn't correct. If anyone can point me in the right direction it would be much appreciated.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Aug 17, 2014 #2


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    Science Advisor
    Homework Helper
    2017 Award

    Hello says, and welcome to PF.

    You seem pretty sure the answer is not correct (how do you know?), so I will accept that.

    Why do you use cp if the exercise says the cylinder is rigid ?
  4. Aug 17, 2014 #3
    Hi BvU! Thanks for the welcome. :)

    I ended up getting this problem right. I was using the Cp value of Nitrogen and not the Cv value.

    Problem solved!
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