How to find ksp from absorbance

Determining the Equilibrium Constant of a Chemical Reaction

Purpose To determine the equilibrium constant, Kc (also called Kf) for the formation of a complex ion by measuring equilibrium concentrations of the reacting species involved. Introduction Many methods may be used to determine the equilibrium constant for a given system. A pH meter may be used to determine the acid dissociation constant, Ka, and it is also possible to determine the solubility product equilibrium constant, Ksp. However in this experiment the equilibrium constant, Kc, for the formation of a complex ion will be determined by measuring the concentrations of reactants in the reaction below. The equilibrium expression is also shown below.

Fe3+(aq) + SCN- (aq) FeSCN2+ (aq) 𝐾𝑐 = [𝐹𝑒𝑆𝐶𝑁2+]

[𝐹𝑒3+][𝑆𝐶𝑁−]

The FeSCN2+ complex ion is a strongly colored species; therefore the reaction can be investigated using spectroscopy. In part A, a calibration curve will be constructed using FeSCN2+ solutions of known concentration. Each standard solution contains a large excess of Fe3+ to ensure all SCN- in solution forms the FeSCN2+ complex. Therefore, the concentration of FeSCN2+ at equilibrium is equal to the initial concentration of SCN-. The calibration curve will be used to determine the equilibrium concentration of FeSCN2+ of each sample (prepared in part B) containing similar concentrations of Fe3+ and SCN- at equilibrium. In part B, mixtures containing similar concentrations of Fe3+ and SCN- will be prepared. Each solution will be allowed to reach equilibrium such that the following mass balance equations (1) and (2) are valid. With these equations and the initial concentration of the two ions the equilibrium concentrations can be determined.

[Fe3+]initial = [Fe3+]eq + [FeSCN2+]eq (1)

[SCN- ] initial = [SCN- ] eq + [FeSCN2+] eq (2)

An alternate and probably more familiar way to determine equilibrium concentrations is the ICE box method. The ICE box method uses the same equations as above but arranges them in a chart like format.

Fe3+(aq) + SCN- (aq) FeSCN2+ (aq)

Initial I

Change 

Equilibrium Eq

The initial concentrations of iron Fe3+ and thiocyanate SCN- ions can be easily determined from the concentration of the stock solutions. The absorbance of the iron thiocyanate ion will be determined by spectroscopy and compared with the calibration curve prepared in part A to determine the equilibrium concentration of the ion. The equilibrium concentrations can be found using the ICE box chart.

Short Review of Beer’s Law & Spectroscopy Beer’s Law is defined below in equation: A = a b c In the equation, A is the measured absorbance of the sample, b is the path length of light through the sample, c is the concentration of the sample, and a is a constant that depends on both wavelength and substance. A linear calibration curve, or Beer’s Law plot, of absorbance (y-axis) versus concentration (x-axis) can be constructed from the absorbance and concentration data of standard solutions. A prepared calibration curve can be used to determine the concentration of an unknown solution from its measured absorbance. Experimental Procedure: General Instructions – Read this BEFORE proceeding to Parts A and B. 1. The SpectroVis Spectrometers and LabQuest units will be used for this experiment.