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Discovering a Change in Equilibrium Constant with Change in Ionic Strength: An Empirical Laboratory Experiment for General Chemistry

Students are challenged to investigate the hypothesis that an equilibrium constant, K c , measured as a product and quotient of molar concentrations, is constant at constant temperature. Spectrophotometric measurements of absorbance of a solution of Fe 3+ (aq) and SCN - (aq) treated with different amounts of KNO 3 are made to determine K c for the formation of FeSCN 2+ (aq). Students observe a regular decrease in the value of K c as the concentration of added KNO 3 is increased.