pKa
Summary
pKa is −log Ka, where Ka is the acid dissociation constant, so a lower pKa means a stronger acid. The most testable point: when pH = pKa, the acid and its conjugate base are present in equal concentrations, as at the half-equivalence point of a titration.
Detail
Meaning: pKa = −log₁₀ Ka. Ka measures how far HA ⇌ H⁺ + A⁻ lies toward products, so a larger Ka gives a smaller pKa and a stronger acid. Each drop of one pKa unit is a tenfold increase in Ka. If pH < pKa, the protonated form HA predominates. If pH > pKa, the deprotonated form A⁻ predominates.
Equation: pH = pKa + log([A⁻]/[HA]). Here pH and pKa are dimensionless, and [A⁻] and [HA] are concentrations in mol/L (only their ratio matters). For a conjugate acid-base pair at 25 °C, pKa + pKb = 14.
On the MCAT: Titration curves are the usual setup. The pKa is the pH at the midpoint of a flat buffer region (the half-equivalence point), and a polyprotic acid shows one such region per pKa. A buffer works best within about one pH unit of its pKa. Amino acid questions give side-chain pKa values and ask for the dominant charge state at a given pH. Organic questions ask you to rank acids by pKa, where electronegative atoms, induction and resonance stabilization of the conjugate base lower the pKa.
Common confusion: A higher pKa does not mean a stronger acid. It means a weaker acid, because less dissociation means a smaller Ka. For a base, use the pKa of its conjugate acid: the higher that pKa, the stronger the base.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.