Question

Phosphoric acid is a triprotic acid:

H3PO4(aq) +H2O(l) ↔ H3O+(aq) + H2PO4-(aq) Ka1 = 7.5 x 10-3
H2PO4-(aq) +H2O(l) ↔ H3O+(aq) + HPO42-(aq) Ka2 = 6.2 x 10-8
HPO42-(aq) +H2O(l) ↔ H3O+(aq) + PO43-(aq) Ka3 = 4.8 x 10-13

Determine whether sodium monohydrogen phosphate (Na2HPO4) is neutral, basic, or acidic.

First, what is its Ka when it acts as an acid?

Second, what is its Kb when it acts as a base?

Answer 1

Na2HPO4, the salt, in aqueous solution is basic in that the pH > 7.0. WHY?
HPO4^2- + HOH --> H2PO4^- + OH^- and the OH^- provides the basicisity.
As an acid, HPO4^- + H2O ==> PO4^3- + H3O^+ and from the problem ka3 = 4.8E-13.
As a base the reaction is as above HPO4^2- + HOH --> H2PO4^- + OH^-
KaKb = Kw. You know Ka and Kw, solve for Kb. Kw = 1E-14 if you've forgotten.