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Discussion in 'MCAT Study Question Q&A' started by GomerPyle, Sep 8, 2012.

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  1. GomerPyle

    GomerPyle 7+ Year Member

    Dec 27, 2007
    This is probably a stupid question. Stems from AAMC 5 number 15: The pka for the dissociation of H2PO4 to HPO4 is 6.7. What is the initial ratio of [HPO4]/[H2PO4] in the buffer solution of experiment 1? (experiment 1 states the solution was buffered at PH 8.7)

    I thought in a buffer, the acid and conj. base concentrations are equal, but in the answer solution, the henderson hasselbach equation is used to calculate the ratio (which I thought would have been 1 in a buffer). What am I not understanding in this concept? Thanks.
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  3. nashaiy


    Jul 26, 2012
    The best buffer solution is one in which the ratio of the conjugate base to the acid is 1:1. In that case, the pH = pKa since log([A-]/[HA]) = 0. This is the exact middle point of the flat region on the titration curve and corresponds to the half-equivalence point. However, a buffer solution can be any combination of acid to conjugate base usually up to a ten fold increase in one over the other. This means that the pH of a buffer solution is equal to the pKa +/- 1 pH point (typically but not necessarily). To clarify, a buffer solution can exist even if the acid's concentration is greater the conjugate base concentration or vice versa. This represents the range you see on the flat portion of a titration curve. Usually, past a 10 fold increase of either concentration, you get a sharp increase in pH which means the solution is no longer a buffer.

    In the example you presented, the pH is given to be 2 points above the pKa of the acid. Thus, the conjugate base concentration is 100x the acid concentration. Is that the correct answer?
  4. chemtopper

    chemtopper online organic/general chem/MCAT/DAT tutor 7+ Year Member

    Mar 3, 2009
    Buffers are different capacities and a good buffer is the one in which pH ~pKa
  5. BerkReviewTeach

    BerkReviewTeach Company Rep & Bad Singer Exhibitor 10+ Year Member

    May 25, 2007
    SDN Exhibitor
    Great explanation nashaiy!

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