Open the Beer s Law Lab simulation on your laptop or tablet:

Transcription

1 Name: BEER S LAW LAB Open the Beer s Law Lab simulation on your laptop or tablet: 1. Explore the controls in the Concentration screen of this sim for a few minutes. 2. Can the cation of the solute affect the color of a solution? Can the anion of the solute affect the color of a solution? Provide supporting evidence for each of your responses. 3. How does the concentration of a solution affect its appearance? Does changing the concentration change the color, or the intensity of the color? 4. A beverage company is having trouble with production. The color of their drink mix is supposed to be light red (consistent with a 0.80 M solution), but that is not always the case. What is going wrong? Provide a plausible reason for each observation. The color intensity is to too low, it is too pale. The solution color is wrong! It looks blue. The solution started out with the correct color intensity, but over time the appearance changed, becoming darker. The employees are sure nothing was added to the open vat. BEER S LAW LAB 1

2 5. Explore the controls in the Beer s Law screen for a few minutes. Note: A spectrometer measures the amount of light absorbed by a sample in comparison to the amount emitted by the light source. In this tab you can investigate how a UV-Vis spectrometer functions. When a solution becomes more concentrated it looks darker (the color intensity increases). A spectrometer allows you to measure and quantify this phenomenon. 6. Select the drink mix and direct green light though the solution. Where is the green light most intense? Where is the green light least intense? Where is the intensity of the light changing? 7. Investigate the intensity of the green light passing through the drink mix and graph your results. Transmi ance (%) Absorbance Drink mix concentra on (mm) Drink mix concentra on (mm) 8. When green light (wavelength 508 nm) passes through 1 cm of a drink mix solution the absorbance is What is the concentration of the drink mix solution? If the absorbance = 0.60, what is the concentration? If the absorbance = 0.30, what is the concentration? Each time the concentration is cut in half, what happens to the concentration? BEER S LAW LAB 2

3 9. The output from a spectrometer for many wavelengths is called a spectrum (plural is spectra). A UV-Vis spectrum is shown for a KMnO4 solution, with data generated in the simulation. Collect a few data points; check that you get the same absorbance values. Absorbance Absorbance Spectrum for 800 mm KMnO Wavelength (nm) Label the regions corresponding to the primary colors red, green, and blue. What color is the KMnO4 solution? Which primary colors are absorbed by the KMnO4 solution, and which are not? 10. Now use the sim to collect data and sketch the spectrum for a different solution. Absorbance Iden fy the Red, Green & Blue Regions (primary colors) Wavelength (nm) What color is this solution? Which primary colors does it absorb? Which ones does it not absorb (these are transmitted)? BEER S LAW LAB 3

4 Absorbance 1. How does Concentration affect how much light is absorbed and transmitted through the solution? INVESTIGATING ABSORPTION AND CONCENTRATION 1. Predict what a graph of absorbance versus concentration would look like. Sketch your prediction. Prediction Concentration 2. Choose a solution from the simulation and measure the Absorbance for different concentrations on the preset wavelength setting. Data from the Simulation Concentration M Abs 3. How does your second graph compare to your prediction? BEER S LAW LAB 4

5 4. Based on Beer s Law (A = lc, A = absorbance, = molar absorptivity, l = pathlength and C = concentration), do you expect using different wavelengths of light would change the way your previous graph looks? Why or why not? INVESTIGATING ABSORPTION AND WAVELENGTH 1. a. Compare three solutions of different colors with the same pathlength (width of container). Preset Wavelength: Simulation default setting Variable Wavelength: Set to same color as solution Solution Solution Color Beam Color Value (nm) Abs Beam Color Value (nm) Abs b. What combinations give the most absorbance? Why? c. How are beam color, solution color, and absorbance related? BEER S LAW LAB 5

6 Absorbance 2. a. Choose a solution and keep concentration and pathlength constant as you graph the absorbance for different wavelengths. Solution: (nm) Abs Wavelength (nm) b. What is the value for the preset wavelength for your solution? Mark this point on your graph. c. Why do you think the preset wavelength is the best wavelength to use for this solution? 3. Compare your absorbance spectrum sketch with a group that chose a different solution. Would you use the same wavelength of light to do spectroscopy experiments with different colored solutions? Why or why not? 4. In a lab experiment monitoring the change in concentration of a reddish-brown substance, FeNCS 2+, a wavelength of 455 nm is used. a. Does this wavelength agree with your conclusions about beam color, solution color, and absorbance above? Why or why not? b. What other wavelengths might you consider using for FeNCS 2+ spectroscopy? BEER S LAW LAB 6