Acetic acid is a common weak acid. Its equilibrium dissociation, written in both Arrhenius and Brønsted form, looks like this:

CH₃COOH(aq) ⇄ CH₃COO^-(aq) + H⁺(aq)

CH₃COOH(aq) + H₂O(ℓ) ⇄ CH₃COO^-(aq) + H₃O⁺(aq)

The equilibrium in this system lies far to the left. The K_c expression would be:

Equilibrium constants for the dissociation of weak acids are generally known as K_a values. Parallel expressions for weak bases are called K_b values.