Determining the Concentration of a Solution: Beer s Law

Transcription

1 Determining the of a Solution: Beer s Law AP Chemistry Lab Some solutions have color because they absorb some, but not all of the colors of light that hit them. For example, copper solutions appear blue because they absorb most, or all, of the orange, red and yellow light that hits them. As a result most of the light that is reflected or passes through is blue (with some green and purple). In all cased the color that we see is the opposite (on a color wheel) of the color that is most absorbed. This absorption occurs through when electrons change energy levels. Light with the right energy hits an ion of copper in solution. The energy is absorbed and an electron jumps to a higher level. When the electron drops back down, the light is reemitted in a random direction (as opposed to the direction the photon was traveling). This results in a decrease in the amount of light of that color that emerges from the opposite side of the solution. Different ions can absorb different wavelengths of light and so have different colors. Figure 1 The more ions that are present in solution, the more light can be absorbed. As a result, the amount of light that is absorbed (called absorbance) is directly proportional to the concentration of the solution. In other words a graph of absorbance v. concentration will give a straight line. If we plot absorbance v. concentration for a of solutions whose concentration is known (called standard solutions), we can then use the graph to determine the concentration of an unknown by plotting its absorbance. When you graph absorbance vs. concentration for the standard solutions, a direct relationship should result. The direct relationship between absorbance and concentration for a solution, shown in Figure 1, is known as Beer s law. In this lab, you will be using a Colorimeter (a side view is shown in Figure 2) to measure the absorbance of solutions. A colorimeter (see diagram) passes light of a given color through a small container of solution, called a cuvette. Figure 2 The solution we will be using is green and you will therefore be passing red light through it. In this experiment, red light from the LED light source will pass through the solution and strike a Page 1 of 5

2 AP Chemistry Lab photocell. A higher concentration of the colored solution absorbs more light (and transmits less) than a solution of lower concentration. The Colorimeter monitors the light received by the photocell as percent transmittance. OBJECTIVES In this experiment, you will Prepare and test the absorbance of five standard nickel (II) sulfate solutions. Calculate a standard curve from the test results of the standard solutions. Test the absorbance of a nickel (II) sulfate solution of unknown molar concentration. Calculate the molar concentration of the unknown NiSO 4 solution. MATERIALS LabPro or CBL 2 interface TI graphing calculator Vernier Colorimeter one cuvette five mm test tubes two 10 ml pipets or graduated cylinders two 100 ml beakers PROCEDURE 1. Obtain and wear goggles M nickel (II) sulfate, NiSO 4, solution nickel (II) sulfate, NiSO 4, unknown solution pipet pump or pipet bulb distilled water test tube rack stirring rod tissues (preferably lint-free) 2. Obtain small volumes of 0.40 M NiSO 4 solution and distilled water in separate beakers. 3. Label five clean, dry, test tubes 1-5. Use pipets to prepare five standard solutions according to the chart below. Transcribe the concentration of each solution from your pre-lab assignment. Thoroughly mix each solution with a stirring rod. Clean and dry the stirring rod between uses M NiSO ~10 0 Absorbance 4. Connect a Colorimeter to Channel One of a LabQuest interface. 5.Set up the data-collection mode. a. Start a new file. Page 2 of 5

3 Determining the of a Solution: Beer s Law b. To select MODE, tap on the mode screen with the stylus. c. Select EVENTS WITH ENTRY from the SELECT MODE menu. Enter an appropriate unit. d. Select OK to return to the Main screen. 6. Set up the interface for the Colorimeter. a. Prepare a blank by filling an empty cuvette ¾ full with distilled water. Place the blank in the cuvette slot of the Colorimeter and close the lid. b. Set the wavelength on the Colorimeter to 635 nm, press the CAL button, and proceed directly to Step You are now ready to test the five standard solutions. a. Select START b. Remove the cuvette from your Colorimeter and pour out the water. Using the solution in Test Tube 1, rinse the cuvette twice with ~1 ml amounts, and then fill it ¾ full. Wipe the outside with a tissue, place it in the Colorimeter, and close the lid. c. When the value displayed on the calculator screen has stabilized, press Keep. Enter as the concentration in mol/l. The absorbance and concentration values have now been saved for the first solution. d. Discard the cuvette contents as directed by your instructor. Using the solution in Test Tube 2, rinse the cuvette twice with ~1 ml amounts, and then fill it ¾ full. After closing the lid, wait for the value displayed on the calculator screen to stabilize and press Keep. Enter 0.16 as the concentration in mol/l. e. Repeat the procedure for Test Tube 3 (0.24 M) and Test Tube 4 (0.32M), as well as the stock 0.40 M NiSO 4. Note: Do not test the unknown solution until Step 8. f. Press STO to stop data collection. The absorbance and concentration values have now been saved for the standard solutions. g. Examine the data points along the curve on the displayed graph. As you move the cursor right or left, the concentration (X) and absorbance (Y) values of each data point are displayed below the graph. Record the absorbance values in your data table. h. Use the analysis function to calculate the linear regression for your data. i. Save the data on your USB drive. Be sure to print out the graph for your lab report. 8. Determine the absorbance value of the unknown NiSO 4 solution. a. Obtain about 5 ml of the unknown NiSO 4 solution in another clean, dry, test tube. Record the of the unknown in your Data Table. b. Rinse the cuvette twice with the unknown solution and fill it about ¾ full. Wipe the outside of the cuvette, place it into the Colorimeter, and close the lid. c. Monitor the absorbance value displayed on the LabQuest. When this value has stabilized, record it in your data table. d. Dispose of any of the remaining solutions as directed. Page 3 of 5

4 AP Chemistry Lab DATA ANALYSIS 1. Calculate the linear regression (best-fit line) equation of absorbance vs. concentration for the five standard NiSO 4 solutions. Print or sketch a graph showing the data and linear-regression equation for the standard solutions. 2. Determine the concentration of the unknown NiSO 4 solution. Explain how you made this determination. 3. Describe an alternate method for determining the molar concentration of your unknown sample of nickel (II) sulfate solution, using the standard data. DISCUSSION 1. What is the molar absorbtivity constant? 2. Explain what a spectrophotometer is and what it measures. 3. Explore the simulations of a spectrophotometer at a. Based on the simulations, what is the effect of path length on absorbance by a sample? b. What is the effect of concentration of absorbance? 4. When you use a spectrophotometer or colorimeter, should you set the wavelength of light to be the same color as that of the solution, or would a different color be more appropriate? Explain. (There is a relevant simulation at the Davidson College site that might be useful here.) 5. Compare the uses of UV-Vis, infrared and microwave spectroscopy. What kind of information can be obtained from these methods? (Reliable references?) 6. Discuss sources of error and ways to improve the accuracy and/or precision of your results. Page 4 of 5

5 Determining the of a Solution: Beer s Law Determining the of a Solution: Beer s Law Preliminary Lab Assignment 1. a) State Beer s Law in equation form. b) According to Beer s Law, what will be the value of the slope if absorbance is plotted against concentration? 2. The following data were collected when the absorbance of a series of solutions containing NiCl 2 were measured: M NiSO4 Absorbance (A) a) Complete the table by calculating the concentration of NiCl 2 in each solution. Show a sample calculation. b) Using spreadsheet or graphing software, prepare a graph in which the absorbances given in the table are plotted against the concentration of nickel(ii) chloride. Draw the best fit line and determine its slope. Print out the graph. c) The absorbance of a solution of unknown concentration is found to be What is the concentration of this solution? 3. Complete the following table and copy it into your lab notebook; you will need this information to complete the lab! Include a sample calculation M NiSO ~10 0 Page 5 of 5