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Understanding dissociation constants

  1. May 6, 2012 #1
    Ka = [H+][A-]/[HA]

    Is the term "[HA]" referring to its initial concentration or its equilibrium concentration?
     
  2. jcsd
  3. May 7, 2012 #2

    Borek

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    Staff: Mentor

    All concentrations are equilibrium concentrations. Initial concentration of acid (sometimes called formal or analytical) is [HA]+[A-].
     
  4. May 7, 2012 #3
    OK, another question: why is it necessary to have both [H+] and [A-] on the numerator? It seems like we could have done just as fine using the definition "Ka=[A-]/[HA]" or "Ka=[H+]/[HA]" instead of "Ka=[H+][A-]/[HA]". Is this conventional or am I missing something?
     
    Last edited: May 7, 2012
  5. May 8, 2012 #4

    Borek

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    Staff: Mentor

    As both H+ and A- are products of the reaction changing concentration of either one shifts the equilibrium, so you need both.
     
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