Analytical Chemistry
posted by Sam on .
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 :(

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?

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.

I got the answer, thanks!

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