We use a rather general derivation, but do not go too deep into the details. The constant K is called reaction constant . We mostly use integers for the stoichiometric factors, but that is not de rigeur. Real life is different. The various c i may be the number of mols, the absolute number of particles, or the concentration relative to some fixed value - it doesn't matter as long as the same definition is used throughout.
Lets write it down.
Now comes a decisive step. However you wiggle - if the concentration of O 2 changes some, the concentration of H 2 will change twice as much. It is the equilibrium condition we had before for particles not reacting with each other when we looked at the meaning of the chemical potential. C i because we are treating equilibrium now by definition. Going through the mathematical motions now is easy.