H2AsO−4 is amphiprotic, with a Ka of 1.7×10−7 and a Kb of 1.8×10−12. An aqueous solution of an H2AsO−4 salt is created. Is the solution acidic, basic, or neutral?

Question

H2AsO−4 is amphiprotic, with a Ka of 1.7×10−7 and a Kb of 1.8×10−12. An aqueous solution of an H2AsO−4 salt is created. Is the solution acidic, basic, or neutral?

Answer 1

Assuming Ka2 = 1.7x10^-7 and Ka3 = 1.8x10^-12 for the following dissociations:

H3AsO4 ==> H^+ + H2AsO4^- Ka1 = some value

H2AsO4^- ==> H^+ + HAsO4^2- Ka2 = 1.7x10^-7

HAsO4^2- ==> H+ + AsO4^3- Ka3 = 1.8x10^-12

Then pH = (pKa2 + pKa3)/2 = (6.77 + 11.74)/2 = 9.3 --> Solution will be basic