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SPECTROPHOTOMETRIC MEASUREMENT OF AN EQUILIBRIUM CONSTANTAlexia CainionCHEM 1212KFall 2019GRAPHICAL ABSTRACTDATA AND RESULTS
Table 1 contains all the data pertaining to reactions 1-5 of this experiment. The concentrations and volumes of the stock solutions of Fe¿3+¿¿¿, SCN¿¿¿¿, and [HNO3]were provided to us by the lab manual, so no calculations were needed to determine those numbers. Based on these numbers provided, we had to calculate the initial concentrations of each of these three chemicals. In order to do this, we used the dilution equation M1V1=M2V2. We are solving for M2since M1is the molarity of the stock solution, V1is the volume of the stock solution used, and V2is the total volume of the resulting solution. When the equation is rearranged, it becomes M2=M1V1V2making it easier to solve for M2. Calculations for Reaction 1: 3+¿3+¿=M1V1V2=(0.00010M)(1.0mL)10.0mL=0.000010M Fe¿M2of Fe¿¿¿=M1V1V2=(0.50M)(5.0mL)10.0mL=0.25M SCN¿M2of SCN¿After for solving for M2for each of the chemicals in the reaction, we can use the M2(initial concentration) of Fe¿3+¿¿¿for reactions 1-5 as the equilibrium concentration of Fe(SCN)¿2+¿¿¿for reactions 1-5. We can do this because the concentration of ¿SCN¿is much higher than the concentration of 3+¿Fe¿in the first five reactions. Therefore, we can assume the equilibrium for these reactions lies very far to the right, and that the reactions go essentially to completion. Therefore, the concentration of 2+¿Fe(SCN)¿can be assumed to be the same as the initial concentration of 3+¿Fe¿used to conduct the reaction.The concentrations of HNO3can be ignored throughout these calculations because the HNO3does not participate in the chemical reaction and is only used as a solvent to make sure the 3+¿Fe¿does not participate.
Calculations for Reaction 1: REACTION 13+¿Fe¿¿SCN¿2+¿Fe(SCN)¿Initial0.000010 M0.25 M0.0 MChange-0.000010 M-0.000010 M+0.000010 MEquilibrium0.00 M0.25 M - 0.000010 M0.000010 MFe(SCN)¿2+¿¿¿I used the data in Table 2 to create a Beer’s law plot (calibration curve) in order to relate absorbance to the equilibriumFe(SCN)¿2+¿¿¿. I then plotted that data from Table 2 with absorbance measure at