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 :(

Analytical Chemistry 
Devron,
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,
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,
I got the answer, thanks!

Analytical Chemistry 
drmathews,
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