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Homework Help: Making a Buffer Solution

  1. Mar 11, 2007 #1
    I have worked through the problem and spent quite a bit of time on it but am not extremely confident in my solution. I would just like someone to check my work. Cheers.

    1. The problem statement, all variables and given/known data

    -Need 0.4 L of an aqueous buffer solution of pH=6.7
    -Have access to 0.02 M HCl, 0.02 M NaOH, 0.01 M C3H4N2, and 0.01 M C3H5N2Cl
    -Will use Imidazole (C3H4N2) as weak base. Will use C3H5N2Cl as conjugate acid.
    -Given Kb for C3H4N2 = 9.0 x 10^-8
    -The buffer must have the capacity to absorb 20 mL of either 0.02 M HCl or 0.02 M NaOH and undergo a pH change of no more than +/- 0.1.

    2. Relevant equations


    3. The attempt at a solution

    (Ka)(Kb) = Kw = 1 x 10-14
    Ka * 9.0*10^-8 = 1 * 10^-14
    Ka = 1.1 x 10^-7

    -log(1.1 x 10^-7) = pKa ~ pH = 6.95

    pH = pKa + log([base]/[acid])

    6.7 = 6.95 + log([base]/[acid])

    [base] / [acid] = 10^-0.25 = 0.56 (reasonably close to 1)

    Use 1.56 x 10^-5 M for [base] and 0.001 M for [acid]

    Buffer concentration = 1.02 x 10^-3

    #mol weak base = (buffer concentration) * (desired final volume) = (1.02 x 10^-3) * 0.4 L = 4.06 x 10^-4 mol

    0.01 M = 4.06 x 10^-4 mol / ? L

    4.06 x 10^-2 L = 40.6 mL C3H4N2

    In a 400 mL beaker, add distilled water to 40.6 mL C3H4N2 until most of the desired final volume is reached. Add HCl or NaOH dropwise until the pH reaches 6.7. Add distilled water to the 400 mL line.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Mar 14, 2007 #2

  4. Mar 15, 2007 #3
    This is my last, desperate call for help. Surely someone can lend a hand?
  5. Mar 16, 2007 #4


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    Homework Helper
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    Gold Member


    I have been trying to think about this one for you. Do you need to use all of those given solutions, or are you permitted to just use 2 or 3 of them?

    My first attempt would be to begin with the imidazole hydrochloride and use it as an acid; find the amount of NaOH base titrant necessary to reach a pH of 6.7. Now, instead of performing that partial neutralization, think of the number of moles of imidazole which would be equivalent; and use correspondingly less amount of moles of the imidazole hydrochloride.

    ...... I know that was not complete, but it might be a start. Take care in case I misjudged anything in that method.
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