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. Using a 10 (or 5) mL volumetric pipet (rinse it with a few mL of stock solution), 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 a total of five readings within the absorbance range 0.05-1, for other dilutions, choose fourother capacities 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, select four smaller volumetric pipets. If not, choose volumes that would best span the absorbance range (if 10 mL diluted to 100 mL had absorbance 0.3, what would you expect 30 mL diluted to 100 mL to have?). 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 (M1V1= M2V2), e.g. if you started with 0.200 g of mix in 1 L of stock solution, your stock concentration has 2.00x102mg 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 R2and 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 in the best-fit calculation. 10. Obtain a second, different flavor packet from your instructor, check its absorbance spectrum on the spectrophotometer the instructor has set up, choose a wavelength to use from among the four offered by the colorimeter and repeat steps 1-9. 11. Clean up all your glassware and both cuvets (use detergent, rinse with tap water, then DI water), discard data from the Vernier, and turn off the Vernier.
DATA SHEET – S OLUTION P REPARATION AND B EER ’ S L AW K OOL -A ID ® F LAVOR 1: _____________________________________ Wavelength : ________ Mass of unopened packet 1 _____________________________ Mass of emptied packet 1 _____________________________ Mass of mix 1 transferred _____________________________ Volume of stock solution 1 _____________________________ Stock solution 1 concentration _____________________________ Show calculations for stock concentration and first dilution’s concentration: Stock solution 1 absorbance _____________________________ Unknown solution 1 absorbance _____________________________ K OOL -A ID ® F LAVOR 2: _____________________________________ Wavelength : ________ Mass of unopened packet 2 _____________________________ Mass of emptied packet 2 _____________________________ Mass of mix 2 transferred _____________________________ Volume of stock solution 2 _____________________________ Stock solution 2 concentration _____________________________ Stock solution 2 absorbance _____________________________ Unknown solution 2 absorbance _____________________________ Kool-Aid flavor 1 Dilution (mL/100 mL)
You've reached the end of your free preview.
Want to read all 16 pages?
- Spring '06
- Mole, DI