Concentration may be determined by taking the number of moles of the reagent or product and dividing it by the total volume of the solution (dimension: M, molarity). There is, however, another method of determining the equilibrium constant, Keq, of a reaction. Spectrophotometric analysis results in the quantitative determination of the oncentration of the product, through the Beer-Lambert's Law. The absorbance of light by a colored solution, such as FeSCN2+, is displayed when the solution is placed in the spectrophotometer.

By using standard solutions to determine the average molar absorptivity of the solution, and given the path length, the concentration of the product of the reaction, the red FeSCN2+ solution may be determined. From that data, and from the initial concentrations of the reactant species, the concentrations of the reactants at may also be determined. With the concentrations of the products determined using In this experiment, the aim is to determine the equilibrium constant, Keq, of the FeSCN2+(aq) reaction by using the Beer-Lambert's Law and the Keq expression.