weak base

Meaning: A weak base is a Brønsted-Lowry proton acceptor that reacts with water to only a small extent, so an equilibrium forms with mostly unreacted base. Ammonia and amines are the standard examples. Its conjugate acid (such as NH₄⁺) is a weak acid. The better a base holds a proton, the weaker its conjugate acid.

Equation: Kb = [BH⁺][OH⁻]/[B], where brackets are equilibrium concentrations in mol/L (Kb is usually written without units). For a conjugate pair, Ka × Kb = Kw = 1.0 × 10⁻¹⁴ at 25 °C, so pKa + pKb = 14.

On the MCAT: A passage lists Kb or pKb values and asks you to rank base strength, or asks for the pH of a weak base solution. Build an ICE table. When x is small compared with the starting concentration C, use [OH⁻] = √(Kb × C), then pOH = −log[OH⁻] and pH = 14 − pOH. In a titration with a strong acid, the solution is a buffer before equivalence. At half-equivalence pOH = pKb (pH = pKa of the conjugate acid). At equivalence the pH is below 7.

Common confusion: Strength is not concentration. A weak base is one that ionizes incompletely, whether the solution is dilute or concentrated. Also, the salt of a weak base with a strong acid (NH₄Cl) makes an acidic solution, because the conjugate acid donates protons to water.