Experimentally, the concentration of the molecule complex [AB] is obtained indirectly from the measurement of the concentration of a free molecules, either [A] or [B].
In principle, the total amounts of molecule [A] and [B] added to the reaction are known.
They separate into free and bound components according to the mass conservation principle:
To track the concentration of the complex [AB], one substitutes the concentration of the free molecules ([A] or [B]), of the respective conservation equations, by the definition of the dissociation constant,
This yields the concentration of the complex related to the concentration of either one of the free molecules