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 Kc expression would be:
Equilibrium constants for the dissociation of weak acids are generally known as Ka values. Parallel expressions for weak bases are called Kb values.