Determination of Solution Concentration from Beer s Law

Transcription

1 Determination of Solution Concentration from Beer s Law Name: Period: PURPOSE: To understand the relationship between the amount of light a solution absorbs and its concentration, and then to use this information to find the molarity of an unknown solution. THEORY: Earlier this semester we learned that certain metal salts emitted light of certain frequencies, or colors, when the electrons in the metal moved from a higher energy level to a lower energy level. Similarly, many compounds absorb light of certain frequencies, causing the electrons to be moved to a higher energy level. Because only some of the white light from the sun or other source is absorbed, the object reflects a specific color. For example, blue jeans are blue because the molecules in the dye of the pants absorb colors other than blue, leaving the light with frequencies corresponding to blue being reflected to your eye. In general, the apparent color of a compound or a solution is the compliment of the color of light that is absorbed. This is illustrated by the color wheel shown below, where complimentary colors are shown opposite of each other on the wheel. Figure I A Color Wheel Showing Color Complements. Using the color wheel above, we see if a solution absorbs the color red, it should appear green, or a blue-green, which are its complimentary colors. The fact that substances only absorb light of certain frequencies can be used by chemists to determine the concentration of dissolved substances. Recall, as we have learned, concentration is a measure of how much dissolved solute exists in a solvent to make a solution. Typically, we measure solution concentration by stating its molarity, which is defined as: molarity (M) = moles solute liters of solution

2 For example, if two moles of salt are dissolved in one L of solution, we would have a 2 molar solution of sodium chloride. Remember that concentration is a ratio, so that if you dissolved 0.5 moles of salt in.250 L of solution, you would still have a 2 M solution. What is the relationship between the concentration of a solution and how much light it absorbs? This relationship is given by Beer s Law. Beer s law is a simple and very important statement that says: The Absorbance of a particular solution, A, is directly proportional to the concentration of the substance absorbing the light, as well as the path length the light had to travel through the solution. Mathematically, this is: A=abc Where a is the coefficient of proportionality (also known as the molar absorbtivity, it depends on the frequency, or color, of light used) b is the path length of the cuvette (how far the light traveled in the liquid), and c is the concentration of the liquid. Thus, if we use the same frequency of light every time in the same cuvette, a graph of A vs. C will yield a straight line! In our lab we will investigate this relationship and use the information gathered from the lab to determine the concentration of an unknown solution. To measure the absorbance, we will use a colorimeter, a device which produces light of specific wavelengths and then measures the absorbance of this light after it has passed through the solution of interest. The solution is held in a rectangular flask called a cuvette. A schematic of the colorimeter is shown below: Figure 2: A Colorimeter. The LED emits light of a specific frequency. This light is partially absorbed by the solution in the cuvette and the remaining light is detected to determine the absorbance. MATERIALS: 10 ml graduated pipet with safety bulb 4 Test tubes 2 plastic cups, one with 50 ml 0.50 M blue soln, another with 50 ml DI H 2 O 1 50 ml beaker Colorimeter with marked cuvette Kimwipes Blue solution of unknown concentration Parafilm PRELAB: Here is an outline of the procedure of the experiment:

3 1) Create solutions of known concentration and measure their absorbance with the colorimeter. If Beer s Law is correct, if we make a graph of Absorbance as a function of concentration, we should get a straight line. 2) Measure the absorbance of one unknown solution. You will then use Beer s Law to determine the concentration. The solutions we will use are made by dissolving a small amount of blue food coloring in distilled water. A standard solution has been prepared for you. This solution has a concentration of 0.50 M. However, in order to create an absorbance curve, we must have more than one solution with we can measure absorbance. Thus, you must prepare three other solutions from the standard. You will do this using dilutions and what you have learned in our discussion of molarity. How do you make a diluted solution of known concentration? To do this we will use a pipet, which is a measuring device designed to deliver (you may notice the letters TD on the pipet) a certain volume of liquid very accurately. In each case you want to make 10 ml of diluted solution, but the concentration of the new solution is up to you. As a guide, I suggest that you make one solution whose concentration is greater than 0.25 M, and solution whose concentration is exactly 0.25 M, and a solution whose concentration is less than 0.25 M. The exact concentrations you use are up to you. However, you must have a method by which to make these solutions ready before you come into lab. Thus in addition to your title, purpose, and other standard prelab info, you should follow this information with a completed table similar to the one below: Solution Number Data Table I: Volumes and Concentrations of Standard Solutions Concentration Initial Volume Concentration of Standard Of Standard Of Diluted Solution Solution Solution Volume Of Diluted Solution M 10 ml M 10 ml M 10 ml M 10 ml Include with the table calculations showing how you obtained the initial volume of the solution you need, as well as the concentration of the diluted solution. You also need to create the following table as well. Here you will write down the absorbance values of each solution you use in the experiment. Data Table II: Absorbance Values of Various Solutions Solution Concentration Absorbance of Solution 0.00 M 0.50 M Concentration of Soln. 1 from above Concentration of Soln. 2 from above Concentration of Soln. 3 from above Concentration of Soln. 4 from above Unknown # End of Prelab:

4 CALCULATIONS: 1) Using Logger Pro, create a graph of absorbance of the solution as a function of concentration. Based on our discussion of proportion, decide on a best curve fit to the graph. Paste the graph into your lab notebook, and prove if the data is directly or inversely proportional using methods you have learned in class. Clearly explain your proof. 2) Using your graph, determine the concentration of your unknown solution. Clearly show all your calculations DISCUSSION: 1) A KMnO 4 solution absorbs strongly in the yellow-green region of the visible spectrum. What is the probable color of the solution? 2) A copper(ii) solution with a concentration has of 0.30M has an absorbance of If the solution obeys Beer s Law, what must be the concentration of another copper(ii) solution if the absorbance is 0.16? 3) Explain why you should never pipet the mouth. 4) Explain why water was used to zero the colorimeter. 5) Why must you never touch the clear sides of the cuvette with your fingers? 6) If you used a cuvette whose length were twice as long, would the absorbance of the solution go up, go down, or remain the same? Explain CONCLUSION: Include in your meaningful conclusion the value of the concentration of your unknown solution.

5 PROCEDURE: BEER S LAW 1) Form a lab group of 4 people and put on your apron and goggles 2) In front of you, you will find 4 test tubes in a rack, 2 small beakers, a large beaker, and your pipet. First take the large beaker and fill halfway with distilled water. 3) Practice using the pipet so that each member of your lab group feels comfortable with both filling the pipet to a certain volume and expelling the reagents. Remember to never hold the pipet by the bulb (there is a good chance the pipet will drop!) 4) Once you feel confident about the pipet, add distilled water to one of the small beakers and 0.50 M Blu solution to the other small beaker. 5) Make your solutions: i) Using the dilution calculations from class, add the amount of 0.50 M Blu you need for your first solution to one of the 4 test tubes. ii) Rinse the pipet with some distilled water and then add the amount of water you calculated to the test tube so that the final volume of the solution is 10.0 ml iii) Cover the test tube with Parafilm and shake to mix the diluted solution iv) Label the test tube with a sticker v) Repeat steps i-iv to make your three other solutions. 6) Go to a colorimeter station. Inside the colorimeter should be a cuvette 3/4 filled with distilled water. 7) Log on to the computer, open Logger Pro, and select Beer s Law from the Chem Labs folder 8) Have the instructor help you calibrate the colorimeter at the red wavelength with distilled water and collect your first data point. 9) Rinse the cuvette with a small amout of 0.50 M standard solution. Then fill the cuvette 3/4 of the way with the standard solution, wipe the clear sides with a Kimwipe, and place the cuvette in the colorimeter. When the absrobance value has Keep stabilized, select and enter the concentration of the standard solution into the dialog box and into your lab book. 10) Rinse the cuvette with a small amount of your first diluted solution, and then fill the cuvette 3/4 of the way with the diluted solution. Wipe the clear ends of the cuvette with a Kimwipe, and place the cuvette into the colorimeter. Close the lid and write down the absorbance in your lab book. In addition, select Keep, and in the dialog box, input the concentration of the diluted solution. 11) Repeat steps 5 and 6 for the other two solutions you must prepare. 12) In this lab we get one data point for free. After you have measured all of your diluted solutions, rinse the cuvette with distilled water and fill 3/4 of the way with DI. Insert

6 the cuvette into the colorimeter and record the absorbance. What concentration of Blu should you input for this? 13) Click Stop and ask the instructor to help you curve fit and print your graph. 14) Rinse your cuvette with a small of amount of unknown solution, fill the cuvette 3/4 of the way with the unknown and read the absorbance. Write down this value in your lab book. 15) Rinse the cuvette and fill 3/4 of the way with DI water. Turn off the colorimeter, log off the computer, and clean up your lab station. 16) Remove and return your apron and goggles and return to your desk.