Which acid would donate exactly three protons on complete ionization?

Which acid would donate exactly three protons on complete ionization?

• A. Phosphoric acid
• B. Sulfuric acid
• C. Nitric acid
• D. Acetic acid

Answer: (A)

Some acids can donate more than one proton because they have several acidic hydrogens; the number of protons an acid can release corresponds to how many ionizable hydrogens it contains. Phosphoric acid has three acidic hydrogens, so on complete ionization it can release three H+ to form PO4^3−. The stepwise ionization goes H3PO4 → H+ + H2PO4−, then H2PO4− → H+ + HPO4^2−, and finally HPO4^2− → H+ + PO4^3−. In contrast, sulfuric acid can release two protons (it’s diprotic, with the second dissociation still occurring but there isn’t a third removable hydrogen), while nitric acid and acetic acid each have only one acidic hydrogen and are monoprotic. Therefore, the acid that would donate exactly three protons on complete ionization is phosphoric acid.

Some acids can donate more than one proton because they have several acidic hydrogens; the number of protons an acid can release corresponds to how many ionizable hydrogens it contains. Phosphoric acid has three acidic hydrogens, so on complete ionization it can release three H+ to form PO4^3−. The stepwise ionization goes H3PO4 → H+ + H2PO4−, then H2PO4− → H+ + HPO4^2−, and finally HPO4^2− → H+ + PO4^3−. In contrast, sulfuric acid can release two protons (it’s diprotic, with the second dissociation still occurring but there isn’t a third removable hydrogen), while nitric acid and acetic acid each have only one acidic hydrogen and are monoprotic. Therefore, the acid that would donate exactly three protons on complete ionization is phosphoric acid.