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Homework Help: Using the Henderson-Hasselbalch equation to find solution buffers.

  1. Mar 16, 2012 #1
    1. The problem statement, all variables and given/known data
    As a technician in a large pharmaceutical research firm, you need to produce 100.0 mL of 1.00 M potassium phosphate buffer solution of pH = 7.14. The pKa of H2PO4- is 7.21.

    You have the following supplies: 2.00 L of 1.00 M KH2PO4 stock solution, 1.50 L of 1.00 M K2HPO4 stock solution, and a carboy of pure distilled H2O .

    How much 1.00 M KH2PO4 will you need to make this solution?

    C(a) represents the volume of the conjugate acid (in milliliters), and C(b) represents the volume of the conjugate base (in milliliters).

    2. Relevant equations
    C(a) + C(b) = 100.0 mL
    C(b) / C(a) = calculated ratio (0.85)
    C(b) = C(a) x calculated ratio (0.85)

    C(a) + ( C(a) x (0.85) ) = 100.0 mL

    C(a) = 100.0 / (1+(0.85)) mL

    3. The attempt at a solution
    I just for the life of me can't understand the section after the base/acid ratio is found.
    7.14 = 7.21 + log [base]/[acid]
    7.14 = 7.21 + log x
    logx = pH - pKa (-.07)
    x = 0.85 [base]/[acid] ratio

    But after that I just honestly have no idea, I did a similar problem in class today but I just can't understand it now. I just would like to know what values belong where in the C(a) and C(b) equations and how to get those numbers so I can solve it myself. Thanks.
    Last edited: Mar 16, 2012
  2. jcsd
  3. Mar 17, 2012 #2


    User Avatar

    Staff: Mentor

    You add concentrations, yet you get volume as the answer? Correct this equation and you should be able to solve the problem.
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