Question

why we may assume [SCN-]0= [FwSCN2+] for the standard mixtures, but may not make the same assumption for the equilibrium mixtures in the laboratory of evaluating the equlibrium constant for the formation of the Iron (III) thiocyanate complex by spectrophotometry?

Answer 1

Because it's the equilibrium mixture. In the equilibrium mixture you are actually doing the reaction, and SCN and Fe lose something (A quantity X). This quantity X is the one that'll appear as the concentration of FeSCN complex. And because it's an equilibrium, you know that equilibrium reactions are not completed. They just remain in equilibrium with some reactants and products interacting between them for a time = t. At the beggining of reaction you may assume the same concentration, because the reaction is not taking place yet.

So, what do you have to do? measure the absorbance of the complex, and from there, you'll have the concentration of the complex (The quantity X) and then, calculate Kc.

Hope this helps