Understanding dissociation constants

  • #1
Ka = [H+][A-]/[HA]

Is the term "[HA]" referring to its initial concentration or its equilibrium concentration?
 

Answers and Replies

  • #2
Borek
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All concentrations are equilibrium concentrations. Initial concentration of acid (sometimes called formal or analytical) is [HA]+[A-].
 
  • #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:
  • #4
Borek
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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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