Beer\'s Law Lab - Beer\'s Law, 2019-2020 academic year PDF

Preview

Summary

Beer's Law, 2019-2020 academic year...

Description

Experiment 4: Beer’s Law. Purpose The purpose of this lab is to use our understanding of Beer’s Law and spectroscopic techniques to measure the concentrations and absorbances (including uncertainty) of a solute in a solution, as well as analyze it chemically. Introduction Beer’s Law is an equation that relates the attenuation of light to the properties of a material. The law states that the concentration of a chemical is directly proportional to the absorbance of a solution. It is also known as the Beer-Lambert law, the Lambert-Beer law and the Beer-LambertBouguer law. The reason there are so many names is because there is more than one law involved. In this lab, we will find the absorption spectrum of different permanganate solutions, then determine the wavelength that shows the strongest absorption of all. Procedure Part A 1. Fill a 600mL beaker with RO water, add a stir bar. Heat to a boil. 2. Put 40 mL of unknown Mn2+ solution in 100 mL flask, using a burette. 3. Dispense 5 mL of phosphoric acid into the flask, swirl. 4. Weigh out 0.3-0.5 g of potassium periodate into the flask. 5. Wash out the chemicals into the flask. 6. Heat the flask in the boiling water bath, for about 10 min. 7. Remove flask, leave on bench to cool. 8. Place flask in cold water bath. 9. Fill the flask up to the mark with water. 10. Put rubber stopper on top, shake to mix. the volume should be 100.00 mL. Part B 1. 2. 3. 4. 5. 6.

Label 4 50 mL beakers 1,2,3,4. Dispense 10 mL of KMnO4 in each beaker. Start logger pro and plug the spectrovis into the computer. Fill a cuvette ¾ full for each solution. Place a cuvette containing water into the spectrovis and calibrate it. Place a cuvette containing solution into spectrovis, click collect, then stop.

Part C 1. 2. 3. 4.

Set the spectrometer for Beer’s Law (absorbance/concentration). Record the max in your notebook. Enter the concentration manually into events with entry, do for all 4 concentrations. Click on linear fit for a line of best fit.

5. Check the box show uncertainties. Part D 1. Rinse the cuvette with the cooked unknown solution, then fill ¾ full. 2. Wipe the outside of the cuvette, place in the spectrovis. 3. Read the absorbance value displayed in the bottom left corner of logger. Observations   

Solutions were purple coloured Absorbance peaks at wavelengths between 520 and 560. Part C: absorbance at 545.4 nm: 0.813

Questions 1. Beer’s Law (Equation 24 ) shows a straight line equation when the absorbance versus concentration is plotted, as you did. Use the straight line fit on your graph to determine the extinction coefficient ε for your compound.

2. Use your measured absorbance to determine the concentration of the unknown solution. First calculate the concentration of the solution in the 100 mL volumetric flask, then, remember that you used 35 mL of the stock and diluted it to 100 mL, calculate the concentration in the stock bottle. Use the uncertainties from the graph to find the uncertainty in your unknown solution concentration by propagation.

3. 1. 2. 3.

List 3 sources of systematic error that you may have encountered in this lab. Spectrovis: smudging of fingerprints could have impacted the results. Calibration: it was possible it was not recalibrated, causing inconsistency. Volume not consistent: not all cuvettes were filled with exactly the same volume.

References First Year Lab Manual. (2019). Kingston, ON: Department of Chemistry.

Helmenstine, A. M. (2019, June 24). Learn About Beer's Law. Retrieved from https://www.thoughtco.com/beers-law-definition-and-equation-608172.; used in introduction (n.d.). Retrieved from http://www.chem.ucla.edu/~harding/IGOC/B/beers_law.html....