Posted by Sam on Tuesday, February 26, 2013 at 4:13pm.
What fraction of piperazine (perhydro1,4diazine) 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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