c. Choose Calibrate CH1 : Colorimeter from the Experiment menu, then click . d. Turn the wavelength knob on the Colorimeter to the “0% T” position. e. Type “0” in the edit box. f. When the displayed voltage reading for Reading 1 stabilizes, click . g. Turn the knob of the Colorimeter to the Blue LED position (470 nm). h. Type “100” in the edit box. i. When the voltage reading for Reading 2 stabilizes, click , then click . 6. You are now ready to collect absorbance data for the standard solutions. Click to begin data collection. Note : Take readings within 4 minutes of preparing the mixtures. a. Empty the water from the cuvette. Using the solution in Beaker 1, rinse the cuvette twice with ~1 mL amounts and then fill it ¾ full. Wipe the outside with a tissue, place it in the Colorimeter, and close the lid. Wait for the absorbance value displayed in the Meter window to stabilize. Click , type the concentration of FeSCN 2+ (from your pre-lab calculations) in the edit box, and press the ENTER key. b. Discard the cuvette contents as directed. Rinse and fill the cuvette with the solution in Beaker 2. Follow the procedure in Part a of this step to measure the absorbance, and enter the concentration of this solution. c. Repeat Part b of this step to measure the absorbance of the solutions in Beakers 3, 4, and 5. d. Click when you have finished collecting data. Click the Examine button, , and record the absorbance values for each data pair.
The Determination of an Equilibrium Constant 7. Click the Linear Fit button, . A best-fit linear regression line will be shown for your five data points. This line should pass near or through the data points and the origin of the graph. (Note: Another option is to choose Curve Fit from the Analyze menu, and then select Proportional. The Proportional fit has a y-intercept value equal to 0; therefore, this regression line will always pass through the origin of the graph). Leave the graph and best fit line displayed and proceed to Step 8. Part II Test an Unknown Solution of SCN – 8. Obtain about 10 mL of the unknown SCN–solution. Use a pipet to measure out 5.0 mL of the unknown into a clean and dry 100 mL beaker. Add precisely 5.0 mL of 0.200 M Fe(NO3)3and 40.0 mL of distilled water to the beaker. Stir the mixture thoroughly. 9. Using the solution in the beaker, rinse a cuvette twice with ~1 mL amounts and then fill it ¾ full. Wipe the outside with a tissue, place it in the Colorimeter, and close the lid. Watch the absorbance readings in the Meter window. When the readings stabilize, record the absorbance value for your unknown in your data table. Remove and clean the cuvette. 10. Determine the concentration of the unknown SCN–solution. a.
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