posted by Sam on .
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 :(
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