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March 29, 2017

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I have a question regarding the titration curve. When the ph is equal pKa that mean there are equal amount of conjugate acid to conjugate base. When a weak acid dissociates for example
H3Po4 H+ H2PO4-
My problem: I don’t understand how the ph rises after the halfway mark ( when ph = pka ) we are producing equal amounts of conjugate base and Hydrogen ions simultaneously
Also, Why is the conjugate base more in concentration after the halfway mark?

  • Chemistry - ,

    H3PO4 ==> H^+ + H2PO4^-
    At the half-way point, you're right.
    (base) = (acid)
    But for the remainder of your question you may be confusing H^+ with the other parts. You are titrating H3PO4 with a base, say NaOH, so
    H3PO4 + OH^- ==> H2O + H2PO4^-
    pH = pKa + log [(H2PO4^-)/(H3PO4)].
    After the half-way point, H2PO4^- is increasing and H3PO4 is decreasing. The pH increases for that reason. There is less acid and more base. I don't see that any H^+ is being produced. In fact, H^+ is being neutralized to H2O.
    I hope I have interpreted your question properly.
    A couple of days ago I helped you with a problem and I suggested you might want to get a second opinion. I have forgotten the problem now(I think it was the carboxylic acid pKa + a pk2) and the question was to find pK2.) but I'm curious as to the outcome. I would appreciate a summary of that work and the outcome.

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