Question

The reaction of thiocyanate (HSCN) with Fe3+ produces a 1:1 colored complex with a maximum absorbance at 447 nm. Fe^3+ + HSCN = FeNCS^2+ + H+ The equilibrium concentration was monitored with a Spectronic 20. Exactly 5.00 mL sample of 1.02 x10-3 M solution of buffered HSCN was mixed with a 5.00 mL of a 1.96 x10-3 M buffered solution of Fe3+. The solution was buffered at a pH = 0.30. A transmittance of 31.1% was measured for this mixture at room temperature. Assuming the molar absorptivity for the FeNCS2+complex is 4700 L/cm-mol and the path length of 0.90 cm, calculate the value of the equilibrium constant for this reaction at room temperature.