Question

1.) A diprotic acid has the following pK values: pK_a1 = 5.50, pK_a2 = 9.50

a.) At what pH does [H₂A] = [HA^- ]?

b.) At what pH does [HA^- ] = [A^2- ]?

c.) Which is the principal species at pH = 3.50, H₂A, HA^- , or A^2- ? Justify your answer using the Henderson-Hasselbach equation.

d.) Which is the principal species at pH = 7.5, H₂A, HA^- , or A^2- ? Justify your answer (don’t use Henderson-Hasselbach equation, this is a half-neutralized diprotic acid).

e.) Which is the principal species at pH = 9.8, H₂A, HA^- , or A^2-? Justify your answer using the Henderson-Hasselbach equation.

Answer 1

The general procedure is to introduce the pKa value in the Henderson - Hasselbach equation and calculate the ratio Base/acid