polyvalent acid or base

Meaning: A polyvalent (polyprotic) acid can donate more than one proton per molecule, such as H₂CO₃ or H₃PO₄. A polyvalent base can accept more than one proton, such as CO₃²⁻ or PO₄³⁻. Protons leave one step at a time, each with its own Ka, and later protons are generally harder to remove, so Ka1 > Ka2 > Ka3.

Equation: Ka1 = [H⁺][HA⁻]/[H₂A] and Ka2 = [H⁺][A²⁻]/[HA⁻], where brackets are molar concentrations (mol/L), and HA⁻ and A²⁻ are the successive deprotonated forms.

On the MCAT: A titration of a polyprotic acid with NaOH shows several buffer regions and, if the pKa values are well separated, several equivalence points. At each half-equivalence point, pH = pKa for that step. Questions ask which species dominates at a given pH, which pKa applies to a buffer, or how equivalents are counted (equivalents = moles × protons donated).

Common confusion: Polyvalent describes how many protons can transfer, not how strong the acid is. H₃PO₄ is a weak acid, and H₂SO₄ is strong only in its first dissociation. Also, the second Ka is not equal to the first; it is smaller, because the proton must leave a negatively charged species.