Posted by **Sam** on Tuesday, February 26, 2013 at 4:13pm.

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 -
**Devron**, Tuesday, February 26, 2013 at 4:45pm
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 -
**Devron**, Tuesday, February 26, 2013 at 5:48pm
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 -
**Sam**, Tuesday, February 26, 2013 at 5:52pm
I got the answer, thanks!

- Analytical Chemistry -
**drmathews**, Tuesday, February 26, 2013 at 9:15pm
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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