Help with spectrophotometric determination of equilibrium constant?

Apr 2012
So we had this experiment, we used Fe3+ and SCN- to produce [Fe(SCN)]2+, and I know how to get the best fitting line for the calibration, but how do I get the equilibrium concentrations of the products and the reactants when determining Keq's of unknown solutions? Also, can H2O be used as a blank solution in this experiment?