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Biochemistry pka and pH relationship

  1. Sep 20, 2009 #1
    1. The problem statement, all variables and given/known data
    If a solution of a weak acid contains more HA than A-, then


    2. Relevant equations

    -

    3. The attempt at a solution

    the answer is pH > pKa.

    Okay my question is why is this the answer. I'm trying to understand in what situation you would have the pH greater or less than the pKa and what that would mean in terms of the amount of A- and HA. The Henderson-Hasselbach equation is


    pH = pka + log [A-]\[HA]

    So let's say the pH is 6 and pka is 5. Then your ratio of A-:HA will be 10:1. That's pretty easy. This example would contain more A- than HA. So here, pH > pKa correct? Why is that then the answer to the original question was pH > pKa when you have more HA than A-??
    Like if your pH was 4 and your pKa was 5, then the ratio would be 0.1:1 for A-:HA and mean that there is more HA? Then that would mean pKa>pH. Maybe I'm misinterpreting this!
     
  2. jcsd
  3. Sep 20, 2009 #2

    Borek

    User Avatar

    Staff: Mentor

    Don't waste you time trying to twist the reality. This statement is false.

    --
    methods
     
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