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Thermodynamics - ΔG° and pH

  1. Dec 2, 2008 #1
    1. The problem statement, all variables and given/known data
    The pH of pure water is temperature dependent, for example it is 6.55 at 50 C. Determine ΔG° for the ionization of water at 50C

    2. Relevant equations

    at eqm ΔG=0
    pure water:
    pH 7.00 at 25C

    3. The attempt at a solution
    Not sure if the process is at eqm at 50C

    but if it was you could just use
    and use the pH to get the keq and you end up with 40.5KJ/mol which doesn't seem right

    what is the difference between ΔG° and ΔGf°

  2. jcsd
  3. Dec 2, 2008 #2


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    How exactly did you get to your answer? From the pH find the hydronium ion concentration then deduce the Keq by writing its equation in both equation and reaction form and plugging in the hydronium ion value.
  4. Dec 2, 2008 #3
    [H+] = 10^[-6.55] = 2.8*10^-7 = [OH-] since it is pure water?

    so keq= [2.8*10^-7]^2 = 7.94*10^-14


    = 81.00 KJ mol-

    is that correct, are we sure that it is in an equilbrium at 50C?

    thanks again
  5. Dec 3, 2008 #4


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    We are assuming equilibrium , your work seems fine.
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