Determination of an Equilibrium Constant

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Experiment 14: Determination of an Equilibrium Constant

Objectives:
To study the chemical reaction of Fe3+ and SCN- to produce Fe(SCN)2+ in aqueous solution. To measure concentrations of ions in solution using a spectrophotometer. To determine the equilibrium constant of this reaction at a given temperature.

Procedure:
Part 1: Calibration Curve Using Standard Solutions (Done with a group) 1. Obtain seven clean 150 mL beakers, one beaker per person. 2. Prepare solutions A-1 to A-7 in each beaker, each person preparing one solution. 3. Once all the solutions are prepared, set the wavelength of the spectrophotometer to 450nm. 4. Obtain a cuvette and fill it about half way with solution A-1, the blank sample. Insert the cuvette carefully, as to not get any fingerprints on the sides, into the spectrophotometer and zero the instrument. This will be the only time the spectrophotometer needs to be zeroed. 5. Pour out solution A-1, rinse the cuvette twice with A-2, and record the absorbance for A-2. Do this for the subsequent solutions. 6. Plot the data, absorbance versus concentration. This will be the calibration curve used for Part 2. Part 2: Solutions to Determine the Equilibrium Constant, K

1. Obtain 5 clean 50 mL beakers and prepare solutions B-1 to B-5. 2. Follow steps 3-5 from Part 1, using B-1 as the new blank. 3. Using the calibration curve from Part 1, determine the equilibrium concentration of [Fe(SCN) 2+] from the absorbance measured for B-1 to B-5.

Data Tables:
Part 1
Solutions for Calibration Curve
Solution| 0.200M Fe(NO3) 3| 0.00200M KSCN| 0.1M HNO3| [Fe(SCN) 2+]| Absorption| A-1| 25.0 mL| 0.00 mL| 75.0 mL| 0 M| 0.000|
A-2| 25.0 mL| 1.00 mL| 74.0 mL| 2 x 10-5 M| 0.097|
A-3| 25.0 mL| 2.00 mL| 73.0 mL| 4 x 10-5 M| 0.160|
A-4| 25.0 mL| 4.00 mL| 71.0 mL| 8 x 10-5 M| 0.335|
A-5| 25.0 mL| 6.00 mL| 69.0 mL| 1.2 x 10-4 M| 0.532|
A-6| 25.0 mL| 8.00 mL| 67.0...
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