3. (a) While one student is performing step 2. the other should prepare the first assigned Kool-Aid® stock solution using the mix packet obtained from your instructor. You will
need a 1-L volumetric flask, if available, or a 2-L or 3-L beaker (beakers are lower- precision instruments, typically ±5% of stated volumes). Record the flavor, its labeled mass, and weigh the unopened packet. (b) Keeping all the packaging and keeping that dry, transfer the contents of the packet to the flask or beaker (a powder funnel and a wash bottle filled with DI water may be helpful). Weigh the emptied package, including any paper you may have torn off to open it. Calculate the mass of mix transferred by difference. (c) Dissolve the mix in about ¾ of the target volume of DI water in your vessel, and then bring the volume up to the mark with more DI water. Mix thoroughly (this is now your stock solution). Part B: Beer’s Law Plots, Concentration of Unknowns 4. Rinse one of your cuvets from step 2.(g) three times with your stock solution, dumping the rinses into a ~100 mL waste beaker. Fill the cuvet ¾ full, wipe the sides with a lint- free wipe, check for air bubbles, and read its absorbance vs. your blank cuvet at the wavelength you’ve chosen. 5. Based on your absorbance reading in step 4, make five precision dilutions of your stock solution aiming for absorbances spread roughly between 0.050 and 1.000. Use a 10 or 15 mL volumetric pipet to transfer that amount of stock solution to a 100 mL volumetric flask, dilute to the mark with DI water, mix thoroughly, rinse the cuvet three times, ¾ fill the cuvet, and read its absorbance. 6. As you want five readings within the calibration range given in step 5, for the other dilutions, choose four others from among the available volumetric pipets (perhaps 30, 25, 20, 15, 10, 5, 4, 3, 2, and 1 mL capacities) and 100 mL volumetric flasks. If the absorbance in step 4 was close to 1.000, you’ll want to use smaller volumetric pipets than if the absorbance was less than 0.500. 7. Obtain a sample of unknown concentration from your lab instructor (who might do a known dilution of your stock solution) and read its absorbance. 8. Hand plot on graph paper the absorbance values within the roughly 0.050-1.000 range (except for the unknown) as one series against concentration. Calculate concentration as mg of mix/L of solution. Use your mass of mix (calculated by difference), and the initial volume of your stock solution. Factor in your dilutions (M 1 V 1 = M 2 V 2 ), e.g. if you started with 0.200 g of mix in 1 L of stock solution, your stock concentration has 2.00x10 2 mg of mix/L, and 10 mL of stock solution diluted to 100 mL would then have 20.0 mg of mix/L. Check this plot with your instructor before proceeding; do not include the hand plot in your report. 9. As part of your lab report plot the five calibration values and your blank as one series in a spreadsheet program, include a best-fit (trend) line, displaying its R 2 and best-fit line equation on the plot. Plot the unknown and stock solutions (note if these are off-scale, e.g. blinking or “>”) as a separate series, so that they are not included
You've reached the end of your free preview.
Want to read all 15 pages?