By equivalence point we mean that situation where all the acid has been neutralized. Thus, the conjugate base has been formed and its concentration is
Moles of HCOO^- = 0.050L x 0.10M = 0.005 moles
The total volume is 0.100L and the concentration [HCOO^-] = 0.050M
The solution, of course, does not exist with no acid, HCOOH, present for that would violate the equilibrium constant expression. So it must be that the conjugate base hydrolyzes to produce some of the acid. To do this calculation we need the K_b for the base and we get it from K_a K_b = K_w.
K_b = 5.61 x 10^-11 M
species HCOO^- OH^- HCOOH initial 0.050 0 0 change -x x x equilibrium 0.050 -x x x
[OH^-]^2 = 0.050 x 5.61 x 10^-11 M and [OH^-] = 1.68 x 10^-6 M
pOH = 5.78 and pH = 14 - 5.78 = 8.22