Other acids and bases are only incompletely dissociated into ions in solution, and exist in part as undissociated molecules.

Dissolved hydrogen fluoride, for example, exists as an equilibrium mixture of HF molecules and hydrated H⁺ and F^- ions.

The equilibrium constant for this dissociation usually is written as K_a., for acid dissociation, rather than K_eq:

Hydrogen fluoride is an inorganic acid.