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Homework Help: Biochemistry: Henderson-Hasselbalch Problem

  1. Apr 15, 2012 #1
    1. The problem statement, all variables and given/known data

    How many moles of base would be needed to titrate one mole of this fully protonated peptide to a pH of 4.00?

    This polypeptide is V-A-Y-K-H.

    2. Relevant equations

    Henderson-Hasselbalch: pH= pKa + log[A]/[HA]

    pKa of alpha COOH = 2.4
    The alpha COOH (at the C-Terminus) is the only portion of the molecule that is deprotonated (basic) at pH 4.00, so we use its pKa for the calculation.

    3. The attempt at a solution

    pH = pKa + log [A]/[HA]
    4.0 = 2.4 + log [A]/[HA]
    1.6= log [A]/[HA]
    39.8 = A/HA

    I'm not sure how to go from the ratio of A/HA to the moles of base needed.
  2. jcsd
  3. Apr 16, 2012 #2


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    Staff: Mentor

    You need initial concentration of peptide. Then, knowing final ratio, you can easily calculate concentration of the neutralized peptide and amount of NaOH can be calculated from the neutralization stoichiometry.
  4. Apr 16, 2012 #3


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    Homework Helper
    Gold Member

    What that
    is saying is a way of saying what should be evident from the fact that the pH 4 is substantially (by 1.6 units) above the pK, so the carboxyl will be almost all deprotonated (to within about 2.5% in fact) so the number of moles required is just under 1.

    (Sorry Borek, the question was about moles so you do not need concentration.)
  5. Apr 16, 2012 #4


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    Staff: Mentor

    Yep, missed the "one mole" part. Sorry about that.
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