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

HPO4^2- + HOH ==> H2PO4^- + OH^- as a base since it adds a H^+..
HPO4^2- + H2O --> PO4^3- + H3O^+ as an acid since it donates a H^+.
Ka2 for HPO4^2- = 6.2E-8 from the data so Kb = Kw/Ka = 10E-15/6.2E-8 = 1.6E-7
Which is the larger number?
FYI. Solutions of Na2HPO4 have a pH between 8.0 and 11.0