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Homework Help: Thermodyanmics, thermal properties of matter

  1. Dec 7, 2008 #1
    1. The problem statement, all variables and given/known data
    a beaker contains 200mL of ethyl alcohol at 20degrees. what is the minumum amount of energy that must be removed to produce solid ethyl alcohol..

    the melting/freezing pt is 114degrees Lf=1.09x10^5
    density is 790kg/m^3
    specific heat cap = 2400J/kgK

    2. Relevant equations
    Q=mcdT
    Q=ML
    m=pv
    3. The attempt at a solution
    m=pv=(2.00*10^-4m^3)(790)=0.158kg
    Q1=mcdT
    =0.158(2400)(94)
    =35644.8J
    Q2=ML
    =0.158(1.09*10^5)
    =17222J

    Qtotal=Q1+Q2
    =52866.8J

    i think i did somethign wrong because in the book it says the answer is 68000J can someone help me out i dont know what i did wrong?
     
  2. jcsd
  3. Dec 7, 2008 #2

    Redbelly98

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    That 114 degrees must be wrong. If it were true, it would already be a solid at 20 degrees.

     
  4. Dec 7, 2008 #3

    oooh sorry that 114 is suppose to be -114
     
  5. Dec 7, 2008 #4

    Redbelly98

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    Okay, then do you see the error here:

     
  6. Dec 7, 2008 #5
    oo i added it wrong for dT its suppose to be -114-20 instead of 114-20

    ok thank you
     
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