weak acid

Meaning: A weak acid donates a proton to water incompletely. At equilibrium most of the acid remains as HA, with only a small fraction forming H₃O⁺ and A⁻. The extent of ionization is measured by Ka. Acetic acid and HF are standard examples. Because ionization is partial, [H⁺] is much less than the starting concentration of acid.

Equation: Ka = [H⁺][A⁻] / [HA], where each bracket is an equilibrium concentration in mol/L (Ka is treated as unitless in most MCAT work). pKa = −log Ka. For a weak acid of initial concentration C, [H⁺] ≈ √(Ka × C) when ionization is small.

On the MCAT: Expect questions that rank acids by Ka or pKa, estimate the pH of a weak acid solution, or ask where a titration curve shows pH = pKa (the half-equivalence point, where [HA] = [A⁻]). Weak acid plus its conjugate base forms a buffer, and the Henderson–Hasselbalch equation applies. At the equivalence point of a weak acid–strong base titration, the pH is above 7 because A⁻ is a weak base.

Common confusion: Weak does not mean dilute, and strong does not mean concentrated. Strength is about the extent of ionization, while concentration is how much acid is dissolved. Also, a weak acid has a conjugate base that is stronger than that of a strong acid, but this conjugate base is still usually weak, not a strong base like OH⁻.