conjugate acid
Summary
A conjugate acid is the species formed when a Brønsted-Lowry base accepts a proton, so it has one more H⁺ than the base (NH₃ → NH₄⁺). The stronger the base, the weaker its conjugate acid, and for a conjugate pair Ka × Kb = Kw.
Detail
Meaning: In Brønsted-Lowry terms, a conjugate acid is the species formed when a base accepts a proton. A conjugate pair differs by one H⁺: NH₃ gains H⁺ to give NH₄⁺, its conjugate acid. The stronger the base, the weaker its conjugate acid, because a strong base holds the proton tightly.
Equation: Ka × Kb = Kw = 1.0 × 10⁻¹⁴ at 25 °C, where Ka is the acid dissociation constant of the conjugate acid, Kb is the base dissociation constant of its conjugate base, and Kw is the ion-product constant of water (all unitless). Equivalently, pKa + pKb = 14.
On the MCAT: Expect questions that identify the conjugate pair in a reaction, rank acids from given bases, or predict which way an equilibrium lies: it favors the side with the weaker acid and weaker base. In buffers, pH = pKa when conjugate acid and base concentrations are equal.
Common confusion: Being called an acid does not make it strong. The conjugate acid of a strong base is very weak, and the conjugate acid of a weak base is itself only a weak acid. It is on the product side when the base is a reactant, with one more H and one higher charge.
Sources
- Chemistry: The Central Science
- General Chemistry: Principles and Modern Applications
- Chemistry (Zumdahl)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.