conjugate base

Meaning: In Brønsted–Lowry theory, an acid HA donates a proton and becomes its conjugate base A⁻, which can accept a proton to reform HA. The pair differs by exactly one H⁺. The stronger the acid, the weaker its conjugate base: Cl⁻ barely accepts protons, but acetate is measurably basic. A weak acid's conjugate base can react with water to raise the pH.

Equation: Ka × Kb = Kw, so pKa + pKb = 14 at 25 °C. Ka and Kb are the dissociation constants of HA and A⁻, and Kw = 1.0 × 10⁻¹⁴ at 25 °C; all are equilibrium constants, usually written without units.

On the MCAT: Label conjugate pairs in a reaction, rank bases from pKa values, and predict that equilibrium favors the weaker acid and weaker base. Buffers use the pair: pH = pKa + log([A⁻]/[HA]). At the half-equivalence point of a weak acid titration, [A⁻] = [HA], so pH = pKa.

Common confusion: A strong acid's conjugate base is not a strong base; it is too weak to change the pH in water. The conjugate base is the acid minus one H⁺, not automatically OH⁻. Water is both the conjugate base of H₃O⁺ and the conjugate acid of OH⁻.