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PH of H3PO4 mixed with NaOH

  1. Apr 12, 2007 #1
    1. The problem statement, all variables and given/known data

    What is the pH of the resulting solution when 100.0 mL of 0.10 M H3PO4 is mixed with 200.0 mL of 0.15 M NaOH?


    2. Relevant equations

    Henderson-Hasselbalch, equilibrium constant expressions

    3. The attempt at a solution


    I've thought about this question for over an hour, yet I have still gotten nowhere. I eventually came to the conclusion that the excess number of moles of OH- will dominate. Based on this conclusion, I calculated a pH of 12.82. However, this answer was marked wrong.

    Can anyone lend some insight into this situation?
     
  2. jcsd
  3. Apr 12, 2007 #2

    chemisttree

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    Calculate the number of equivalents of H+ that can be produced by 100 mL of 0.1 M H3PO4. Calculate the number of moles of OH- in 200 mL of 0.15 M NaOH.

    The reaction is:

    H+ + OH- -------> H2O

    It looks like the same question as "What is the pH of a solution of 0.01 moles Na3PO4 in 300 mL water?"
     
  4. Apr 12, 2007 #3
    Thanks for the reply. If you don't mind, could you look over what I have done and see if I am even close to the right track?

    [​IMG]


    I don't know. I still think this is wrong. I just don't know what to do...haha.
     
    Last edited: Apr 12, 2007
  5. Apr 12, 2007 #4

    chemisttree

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    Let me rephrase my original post. How many potential moles of protons can be generated by 0.100 L of of 0.1 moles/liter of H3PO4? Compare this to the number of available moles of OH-.

    After that, consider answering the question, "What is the pH of a solution of 0.01 moles Na3PO4 in 300 mL water?"
     
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