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Homework Help: Another pH

  1. Feb 13, 2009 #1
    1. The problem statement, all variables and given/known data

    [tex]50ml[/tex] of [tex]0.10M Na_3PO_4 + 50ml[/tex] of [tex]0.10M NaH_2PO_4[/tex]

    For [tex]H_2CO_3[/tex]:
    [tex]Ka_1 = 7.5 * 10^{ - 3}[/tex]
    [tex]Ka_2 = 6.2 * 10^{ - 8}[/tex]
    [tex]Ka_3 = 1.0 * 10^{ - 12}[/tex]


    2. Relevant equations



    3. The attempt at a solution

    Since [tex]K_h[/tex] of [tex]Na_3PO_4 > > Ka_2[/tex] , I considered only hydrolysis of [tex]Na_3PO_4[/tex] but it gives wrong answer.
     
  2. jcsd
  3. Feb 14, 2009 #2

    Borek

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    Staff: Mentor

    You have a mixture of a weak acid and its conjugate base - does it ring a bell?
     
  4. Feb 14, 2009 #3
    Actually, there are so many equilibria in this solution I cant figure out what will contribute and the assumption I made(given above) seems to give a wrong answer.
     
  5. Feb 14, 2009 #4

    Borek

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    Staff: Mentor

    Actually my previous comment was not too good :frown:

    Think this way: you have mixed equimolar amounts of Na3PO4 and NaH2PO4. Does it differ from just dissolving Na2HPO4?
     
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