Phosphoric acid, h3po4, is a triprotic acid, for which ka1 = 7.2 × 10–3, ka2 = 6.3 × 10–8 and ka3 = 4.2 × 10–13. What is the value of kb for the hydrogen phosphate anion, hpo42–?

Kb = 1.587x10⁻⁷

Explanation:

The equilibriums of phosphoric acid in water are:

H₃PO₄ ⇄ H₂PO₄⁻ + H⁺ Ka = 7.2x10⁻³

H₂PO₄⁻ ⇄ HPO₄²⁻ + H⁺ Ka = 6.3x10⁻⁸

HPO₄²⁻ ⇄ PO₄³⁻ + H⁺ Ka = 4.2x10⁻¹³

The Kb of HPO₄²⁻ is:

HPO₄²⁻ + H₂O ⇄ H₂PO₄⁻ + OH⁻

This equation is the result of the water equilibrium - Second Ka of phosphoric acid.

And Kb = Kw = Ka

Kb = 1x10⁻¹⁴ / 6.3x10⁻⁸