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Homework Help: Total heat removed from phase change

  1. Feb 7, 2010 #1
    Ethyl alcohol has a boiling point of 78.0°C, a freezing point of -114°C, a heat of vaporization of 879 kJ/kg, a heat of fusion of 109 kJ/kg, and specific heat of 2.43 kJ/(kg · K). How much energy must be removed from 0.584 kg of ethyl alcohol that is initially a gas at 78.0°C so that it becomes a solid at -144°C?

    I know start out by finding the heat removed at the phase change.

    Q1 = Lvm
    = 879(0.584)
    = 513.336 kJ

    Then the heat involved to cool the liquid to −114°C...

    Q2 = cmΔT
    = 2.43(0.584)(78− - 114)
    = 272.471 kJ

    Then the phase change at −114°C...

    Q3 = Lfm
    = 109(0.584)
    = 63.656 kJ

    Adding them all together to get the total heat...

    Q1 + Q2 + Q3 = 513.336 + 272.471 + 63.656 = 849.463 kJ


    But webassign won't take my answer. What did I do wrong?

    Please help. Thanks.


    Edit: I figured out what I did wrong. Didn't pay attention to the final temperature. Got it right this time. :)
     
    Last edited: Feb 7, 2010
  2. jcsd
  3. Dec 9, 2014 #2
    Hi, not sure if you'll be able to see this since this thread was posted 4 years ago but what did you mean by u didn't pay attention to the final temperature? What did you do right?
     
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