[miaou5]

My book tells me that the equilibrium constant for a net reaction made up of two or more steps is the product of the equilibrium constants for the individual steps. Hence, if I were to combine the following 2 reactions:

A  «=»  B + C (Reaction 1)

C + D  «=»  E (Reaction 2)

My Keq would look like

Keq = [A][C][D]/[ B][C][E] = [A][D]/[ B][E]

What I DON'T understand is, why can the [C] values in the numerator and the denominator cancel out? It's not like they're a constant, and at equilibrium, the concentration [C] made by Reaction 1 and the concentration [C] present in Reaction 2 most likely wouldn't be equal. So at least to me, it doesn't make sense to be able to just cross them out. (Hopefully this question makes sense?) Thanks so much in advance!

[Borek]

at equilibrium, the concentration [C] made by Reaction 1 and the concentration [C] present in Reaction 2 most likely wouldn't be equal

If both reactions take place in the same solution you can't have two different concentrations of the same substance.