Which acid is triprotic?

Which acid is triprotic?

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

Answer: (C)

The number of protons an acid can donate defines whether it is monoprotic, diprotic, or triprotic. A triprotic acid can release three protons in three successive steps. Phosphoric acid, H3PO4, has three acidic hydrogens on the molecule, so it can lose three protons: H3PO4 ⇌ H+ + H2PO4−, H2PO4− ⇌ H+ + HPO4^2−, and HPO4^2− ⇌ H+ + PO4^3−. This makes it triprotic because three separate protons can be donated, each with its own roughly distinct pKa value. In contrast, acetic acid donates just one proton (monoprotic), nitric acid also donates one (monoprotic), and sulfuric acid donates two protons (diprotic)—the first dissociation is very strong, giving up the proton readily, while the second is weaker.

The number of protons an acid can donate defines whether it is monoprotic, diprotic, or triprotic. A triprotic acid can release three protons in three successive steps. Phosphoric acid, H3PO4, has three acidic hydrogens on the molecule, so it can lose three protons: H3PO4 ⇌ H+ + H2PO4−, H2PO4− ⇌ H+ + HPO4^2−, and HPO4^2− ⇌ H+ + PO4^3−. This makes it triprotic because three separate protons can be donated, each with its own roughly distinct pKa value.

In contrast, acetic acid donates just one proton (monoprotic), nitric acid also donates one (monoprotic), and sulfuric acid donates two protons (diprotic)—the first dissociation is very strong, giving up the proton readily, while the second is weaker.