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Why deprotonated?

  1. Aug 25, 2009 #1
    Can someone explain to me why the carboxylic acid group and the amino group both have charges on them in neutral water? The pKa for carboxylic acid is around 2-5 depending on its enviroment. I am confused that a pKa of 2.3 would give a ratio of 80000 deprotonated to 1 protonated? If the acid has a pKa it means it is weak, a pKa of 2.3 gives a concentration lower than 1. I assume the majority would be protonated. But all info I read states that the majority is deprotonated at a pH of 7. Can someone explain why to me it is this way? Thank you!
     
  2. jcsd
  3. Aug 26, 2009 #2
    [tex]pK_{a}-pH=log{{[HA]}\over{[A^{-}]}}[/tex]

    The majority is protonated when [tex]pH<pK_{a}[/tex].

    The majority is deprotonated when [tex]pH>pK_{a}[/tex].
     
    Last edited: Aug 26, 2009
  4. Aug 26, 2009 #3

    chemisttree

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    Are you asking why an amino acid is a zwitterion at neutral pH?
     
  5. Sep 2, 2009 #4
    Thanks for the replies! I was having problems with looking at everything relative, because of course if you put a carboxyl group in water, it will make a low pH. What i did not understand is that a pH of 7 has so many hydroxyl groups to completely deprotonate the acid. So yeah it had to do with zwitterions too. Thank you guys.
     
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