If I want to calculate the number of moles ratio of reactants which, in a reversible reaction of equilibrium constant K, will give me a certain yield 'f', what is the mathematical definition of the yield in this case? For instance, in the reaction A + B ⇔ C + D, let's say we can denote extent by ΔA such that K=(C0-ΔA)*(D0-ΔA)/((A0+ΔA)*(B0+ΔA)). Then is yield f=-ΔA/A0 (i.e. 90% yield would suggest that ΔA=-0.9*A0)? The problem becomes that then this is not 90% of everything, because B0 may not be the same as A0, in which case 90% of B0 would not have been converted, only 90% of A0. And if this "90% yield" was done with ΔB instead then our definition would involve 90% of B0 being converted but not 90% of A0, which provides two conflicting understandings of the "90% yield" statement.