Analytical Chemistry

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What fraction of piperazine (perhydro-1,4-diazine) is in each of its three forms (H2A, HA–, A2–) at pH 7.05?

Not sure how to find k1 and k2 needed for the fraction formulas...please help :(

  • Analytical Chemistry -

    looking at the structure, the NH3 groups will have the same pka values (9.731). You will have to use the Henderson Hasselbalch equation (pH=Pka +log(A-/HA) to solve. So, at a pH of 7.05 what is the fraction of dissociation?

  • Analytical Chemistry -

    You can do it using ka=[HA-][H30+]/[H2A] and ka=[A2-][H30+]/[HA-], but I THINK that you can use the Henderson Hasselbalch equation twice and get the same result. I didn't try and plug in the numbers and see.

  • Analytical Chemistry -

    I got the answer, thanks!

  • Analytical Chemistry -

    aH2A = [H+]^2/([H+]^2 + [H+]K1 + K1K2)

    aHA- = K1[H+]/([H+]^2 + [H+]K1 + K1K2)

    aA2- = K1K2/([H+]^2 + [H+]K1 + K1K2)

    KA1 and KA2 can be found elsewhere
    Ka1 = 4.65×10–6 and Ka2 = 1.86×10–10

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