Question

Phosphoric acid, H3PO4, is tribasic with pKa values of 2.14, 6.86, and 12.4. The ionic form that predominates at pH 3.2 is: H3PO4 + H2O ⇌ H3O+ + H2PO4 - ⇌ H3O+ + HPO4 2- ⇌ H3O+ + PO4 3- The answer is H2PO4- Can you explain the concept/reasoning behind this?

Answer 1

At pH 3.2, the predominant ionic form of phosphoric acid is H2PO4-. This is because the first proton has already dissociated to form H3O+ and H2PO4-, and the second proton has not yet fully dissociated to form H3O+ and HPO42-.

Step-by-step explanation:

Phosphoric acid, H3PO4, is a tribasic acid because it has three acidic protons that can dissociate in water. The dissociation of phosphoric acid occurs in a stepwise manner, with each step having a different equilibrium constant (pKa value).

At pH 3.2, the predominant ionic form of phosphoric acid is H2PO4-. This is because at this pH, the first proton has already dissociated to form H3O+ and H2PO4-, and the second proton has not yet fully dissociated to form H3O+ and HPO42-. Therefore, the concentration of H2PO4- is the highest in the solution at pH 3.2.