Question

Phosphoric acid is a polyprotic acid, with p K values of 2.14, 6.86, and 12.38. Which ionic form predominates at pH 9.3

Answer 1

Answer:

HPO₄⁻² predominates at pH 9.3

Explanation:

These are the equilibriums of the phosphoric acid, a tryprotic acid where 3 protons (H⁺) are realesed.

H₃PO₄ + H₂O   ⇄   H₂PO₄⁻   +   H₃O⁺   pKa 2.14

H₂PO₄⁻  +  H₂O   ⇄   HPO₄⁻²  +  H₃O⁺  pKa 6.86

HPO₄⁻²  +  H₂O   ⇄   PO₄⁻³   +   H₃O⁺  pKa 12.38

The H₂PO₄⁻  works as amphoterous, it can be a base and acid, according to these equilibriums.

H₂PO₄⁻ +  H₂O   ⇄   HPO₄⁻²  +  H₃O⁺

H₂PO₄⁻ +  H₂O   ⇄   H₃PO₄  +  OH⁻

pH 9.3 is located between 6.86 and 12.38 where we have this buffer system HPO₄⁻² / PO₄⁻³, where the HPO₄⁻² is another amphoterous:

HPO₄⁻ +  H₂O   ⇄   H₂PO₄⁻  +  OH⁻

HPO₄⁻²  +  H₂O   ⇄   PO₄⁻³   +   H₃O⁺

The media from the two pKa, indicates the pH where the protonated form is in the same quantity as the unpronated form, so:

(6.86 + 12.38) /2 =  9.62

Above this pH, [PO₄⁻³] > [HPO₄⁻²].

In conclussion, at pH 9.3, [HPO₄⁻²] > [PO₄⁻³]