Also if you want to calculate chemical equilibria in the liquid and gas phase, of course, now you've seen chemical equilibrium, so the amount of reaction depends on the composition.

But again, if you know all the energies of the possible states, in the solid, in the liquid and the gas, statistical mechanics shows us that we can calculate the equilibrium between those.

Since the gas and liquid are in equilibrium, therefore we know the chemical potential in the liquid phase too.