7 Before you measure the absorbance of a solution you must calibrate the

7 before you measure the absorbance of a solution you

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7. Before you measure the absorbance of a solution, you must calibrate the colorimeter at the appropriate wavelength. Over simplifying slightly, the calibration “tells” the colorimeter to ignore any absorbance by the solvent, in this case the FeCl3 solution. Press the < or > button on the colorimeter so as to set the wavelength to 565 nm. (The solutions that you are preparing absorb most of the visible light at this wavelength). Rinse the cuvette twice with the acidified 0.02 M iron(III) chloride solution. Discard this rinse into your waste beaker. Fill a cuvette about 3⁄4 full with the acidified 0.02 M iron(III) chloride solution, cap it and then wipe the clear sides with a Kimwipe (tissue). Kimwipes are used instead of paper towels because kimwipes will not scratch the optical surface of the cuvet. Open the lid of the colorimeter and insert the cuvette making sure that the clear sides of the cuvette line up with the arrow at the top of the cuvette slot. Close the lid. Press the CAL button and release. The red light should flash briefly and the absorbance reading on the LabQuest should be 0.000 or 0.001. The colorimeter is now calibrated and ready for use. 8. Rinse the cuvette two times with Solution 1. Use the pipette to transfer the solution to the cuvette, and use a waste beaker for the rinsings. Then fill the cuvette about 3⁄4 full with Solution 1, cap it and then wipe the clear sides with a Kimwipe (tissue). Open the lid of the colorimeter and insert the cuvette making sure that the clear sides of the cuvette line up with the arrow at the top of the cuvette slot. Close the lid. Start collecting absorbance data by tapping the green arrow, ►, on the lower left of the screen. This will be Run 1. The plot should look something like this; do not be alarmed if the absorbance values vary by 0.1 absorbance units:
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4 9. Once the data collection has completed, you need to determine the mean (average) absorbance for Run 1. To determine the mean absorbance, highlight the graph with the stylus, then from the Analyze Menu select Statistics, tap the box next to Absorbance and record the mean absorbance, A mean , in your lab notebook. 10. Before starting a new Run, you should tap the file cabinet icon (next to the box that reads Run 1) with the stylus. This will create a new graph for Run 2. Measure the mean absorbance of Solutions 2 – 5 as outlined in step 8 and 9. Absorbance values should all be different and the maximum absorbance should not exceed 1.0. 11. At the end of this part of the experiment, all solutions may be disposed of in the waste container. Rinse thoroughly all pipettes, flasks, and cuvettes with deionized water. PART B 12. Obtain one aspirin tablet and place it in a 125-mL Erlenmeyer flask. Record the brand and the amount of acetylsalicylic in the tablet that appears on the label. 13. Add about 10 mL of 1 M NaOH(aq) solution and heat the mixture to boiling, taking care that none of the solution splatters or is lost. The tablet may not dissolve completely in the base and the solution may be cloudy. This is OK since all of the acetylsalicylic acid will have dissolved and reacted with the NaOH. Only the fillers in the aspirin tablets are
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