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Homework Help: Determine the change in internal energy of boiling water

  1. Feb 24, 2004 #1
    Determine the change in internal energy of 1 kg of water at 100 degrees C when it is fully boiled. Once boiled this volume of water changes to 1671 Liters of steam at 100 degrees C Assume the pressure remains constantat 1 atm

    things i know
    1 L =1E-3 m3
    1atm = 1.013E5 N/m2
    1 L = 1000 cm3 = 1E-3 m3
    formulsa used
    Q=mLv = 1kg*2.26E6 J/k Q=2260000 J
    W = -P(Vsteam-Vwater) = (1.013E5)*[(1.670 m^3) = - 169272 J

    change in U = Q+W
    2260000-169272
    U = 2090728 J

    are these correct?

    Now i have not took chenistry yet but it wants to know how many moles of water were converted to steam
    I have 2 different answers
    1) 54.6 mols using n = PV / RT= 1.013E5*1670 / 8.31*373.15
    or
    2)18g/mol using the back of the book periodic table (molar mass)
    H2O = 1+1(of hydrogen)+16 (oxygen)
    which is right if somebody can help me befor 300pm today Please
    thanks joe
     
  2. jcsd
  3. Feb 26, 2004 #2

    Tom Mattson

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    Staff Emeritus
    Science Advisor
    Gold Member

    Re: energy

    I haven't checked the numbers, but the procedure is right.

    The "54.6 mols" and "18g/mol" aren't compatible quantities. The first tells you how many moles you have and the second tells you the mass of a mole.

    Sorry I was late.
     
  4. Feb 26, 2004 #3
    Thanks alot for the reply, at least i am starting to get a grasp on this subject. And doing better in the class

    Thanks again Tom


    joe
     
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