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Homework Help: Second equivalence point

  1. Apr 14, 2010 #1
    1. The problem statement, all variables and given/known data

    You have a solution that is buffered at pH =2.0 using H3PO4 and H2PO−4 (pKa1 = 2.12;
    pKa2 = 7.21; pKa3 = 12.68). You decide to titrate this buffer with a strong base. 15.0 mL are needed to reach the first equivalence point. What is the total volume of base that will have been added when the second equivalence point is reached?

    I know that the answer is >30ml but I don't know how to get to that point.

    2. Relevant equations

    PH=(PKa1+PKa2)/2 First equivalence point
    PH=(PKa2+PKa3)/2 Second equivalence point

    3. The attempt at a solution

    I honest don't know what to do, I did try finding the concentrations of OH at the equivalence points using the pH formulas above, but that didn't seem to get me anywhere since those values were tiny and I didn't know how to get mL from it..

    Any help would be much appreciated! Thanks!
  2. jcsd
  3. Apr 16, 2010 #2


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    Staff: Mentor

    Use information about pH to find initial concentrations of H3PO4 and H2PO4-. First end point means you have neutralized all H3PO4. Now you have solution containing initial H2PO4- and H2PO4- from H3PO4 neutralization.

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