Question

What value should you calculate when pka2 and pkb2 are added together?

Answer 1

pKa2 (acid) + pKb2 (conjugate base) = 14 (pKw)
In diprotic acids:
Also, pKa1 + pKb2 = 14 and pKa2 + pKb1 = 14 and pKa1 always smaller than pKa2 because the first proton is always more acidic

Answer 2

 pKa2 and pKb2 = 14. pKa1 is much less than pKa2 this is because the first hence pKa1 and pKa2 they don't equal to 14.

pKa2 contains all dissociated hydrogen (H+) which is being possible.