Question

Ascorbic Acid is an organic diprotic acid, H2A, that is found in many natural materials. KA1=7.94x10^-5 and KA2=1.62x10^-12. If you start with a 0.75 M solution of H2A, what is the concentration of ALL species in the solition at ph=3.8?

I have no idea how to even start this equation. Please help!

Answer 1

This is done just like a monoprotic acid but just do one acid at a time and reason it through. The pH is 3.8 so what is (H^+)? pH = -log(H^+) so (H^+) is about 1.6E-4
............H2A ==> H^+ + HA^-
I..........0.75..1.6E-4...0
C...........-x......x......x
E.........0.75-x. 1.6E-4+x..x

Substitute the E line into Ka1 and solve for x and the others; thata gives you H2A, H^+, and HA^-. That leaves only A^2-.

Now set up Ka2 the same way.
...........HA^- ==> H^+ + A^2-

Substitute the E line into Ka2 expression and solve for A^2-.