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Homework Help: PH in an arsenic acid solution

  1. Oct 8, 2012 #1
    1. The problem statement, all variables and given/known data

    The cid constants for the polyprotic acid H3AsO4 are Ka1=5x10^-3, Ka2=8x10^-
    8, Ka3=6x10^-10. What is the pH of a 0.1M Na2HAsO4 solution?

    2. Relevant equations
    Kb = [H2A]*[OH]/[HA]
    mass balance, charge balance.. etc.

    3. The attempt at a solution

    Well I've tried setting up a bunch of equations but I can't do this. Does anyone have any tips?
  2. jcsd
  3. Oct 8, 2012 #2
    never mind.. I asked a classm8 and he told me about the pH=pka1+pka2 /2 equation.
  4. Oct 9, 2012 #3


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    Gold Member

    For future, you may want to remember that formula as the pH of buffers.
  5. Oct 9, 2012 #4


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    Staff: Mentor

    No, these things are completely unrelated.

    Close, but wrong.
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