Answer:
HPO₄⁻² + H₂O ⇄ PO₄⁻³ + H₃O⁺ Ka3
Step-by-step explanation:
Phosphoric acid, H3PO4 is a dyprotic acid which undergoes in these three succesive ionization reactions.
The last equillibrium is conditionated to the Ka3
H₃PO₄ + H₂O ⇄ H₂PO₄⁻ + H₃O⁺ Ka1
H₂PO₄⁻ + H₂O ⇄ HPO₄⁻² + H₃O⁺ Ka2
HPO₄⁻² + H₂O ⇄ PO₄⁻³ + H₃O⁺ Ka3
It is an acid because it release a proton, to make the [H₃O⁺] rise.
pH of solution will always be < 7
H₂PO₄⁻ and HPO₄⁻² are amphoteric compounds, which means that they can work as an acid (release protons), or base (take protons)