You are analyzing water samples from a local stream and want to use the intense red color of Fe(SCN)²⁺ to measure the concentration of Fe³⁺. You need to know the formation constant (Topic 6I) for this complex to determine the concentration of Fe³⁺.

The molar absorption coefficient, ε, for Fe(SCN)²⁺ at 457 nm is 4.8 * 10³ L · mol^–1 · cm^–1. A solution is prepared with initial concentrations of Fe³⁺ and SCN^– of 0.30 and 0.20 mmol · cm^–1, respectively. The resulting red solution has an absorbance at 457 nm of 0.0474 in a cuvette with path length 1.00 cm. Determine the formation constant, K_f, for Fe(SCN)^–1.

PLAN Then use Beer’s law to determine the equilibrium concentration of Fe(SCN)^–1. Solve for the equilibrium concentrations of Fe³⁺ and SCN^– and substitute into the equilibrium expression to determine K_f.

Fe+ (aq) + SCN(aq) Fe(SCN) + (aq) Kf= [Fe(SCN)+] [Fe+][SCN]