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? ka = 4.8e-13...i guessed and that's correct, but I need to know how to determine Ka when it acts as an acid or ka when it acts as a base? how would i figure it out without guessing?

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? ka = 4.8e-13...i guessed and that's correct, but I need to know how to determine Ka when it acts as an acid or ka when it acts as a base? how would i figure it out without guessing?