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Beer’s Law: Colorimetry of Copper (II) Sulfate

Image of a talking Lab Quest Objective: 1. Use colorimetry to determine the concentration of a copper (II) sulfate solution by constructing a standard curve. 2. Observe factors affecting absorbance and color intensity. 3. Relate observations of color to Beer’s Law. Prerequisite Knowledge:

NSTA Science Process Skills:  Observe:  Measure:  Infer:  Sort:  Predict:  Communicate:

Accessibility: (Specific changes to procedure in blue parentheses)

What You Need:  Labquest – 1 per 2  Colorimeter probe – 1 per 2  Copper sulfate, pentahydrate, powder grade - ~100 grams  1 L Volumetric flask – 1  Distilled water – 1 gallon  KoolAid – 1 packet per class  1 L Beaker – 1  600 mL beaker – 5  Cuvette, plastic – 2 per 2  Test tube rack – 1 per 2  Test tubes, 20 mL capacity – 5 per 2  Graduated cylinder, 10 mL capacity – 1 per 2  Pipets, transfer – 5 per 2  Beaker, 100 mL capacity, for containing distilled water – 1 per 2 Preparation:  Prepare 1 liter of 0.40 M Copper sulfate solution. In a volumetric flask, add 99.874 g of CuSO4•5H2O to 800 mL of distilled water. Stir or swirl until dissolved. Add distilled water to total 1 L.  Prepare two copper sulfate solutions [A] and [B] for use as unknowns by diluting a portion of the 0.40 M Copper sulfate solution with distilled water.  Set colorimeters to 635 nm setting.  Fill the 100 mL beakers halfway with distilled water for student use during the dilutions. What You Do: Introductory Activity: 1. Perform a dilution of some KoolAid in front of students. Direct the students to move as close to you as possible with everyone still being able to see. Begin by adding one package of Koolaid to 750 mL of

water in a 1 L beaker and stirring it. Ask, “What is the concentration now?” [Answers will vary] “Let’s just call it KoolAid strength.” 2. To five other 600 mL beakers add approximately 50 mL, 100 mL, 150 mL, 200 mL, and 250 mL of the KoolAid. Ask, “What is the concentration now?” [same] 3. Fill each of the five beakers with water up to the 500 mL line. Ask, “What is the concentration now?” [less than before, 1/10th, 1/5th, 3/10th, 2/5th, and ½ the concentration as before] 4. Ask, “Do all of these look the same?” [no, some are lighter] 5. Ask for a volunteer to look carefully at the ½ concentration beaker. Ask, “Does this look the same color from the top as it does from the side?” [No.] 6. Ask, “What happens to the color as the solutions get less concentrated?” [gets lighter] 7. Ask, “If I showed you some randomly diluted KoolAid, could you tell me which one of the five beakers it was between?” [Yes] Activity: 1. Show students the Vernier probe. Tell them, “This probe measures color by shooting out a colored laser and measuring how much of it gets absorbed. It turns that into a number between 0 and 1 called absorbance. We’re going to measure this absorbance for some solutions of known concentration, graph it, and then use the graph to figure out the unknown concentration. 2. Show students the plastic cuvette. Say, “This cuvette doesn’t interfere with the laser. It also makes sure we’re looking through the correct distance of liquid since that made such a difference with the KoolAid. Try not to touch the flat part as your fingerprints do interfere with the laser.” 3. Show students how to insert the cuvette and take a reading. The absorbance for the empty or blank cuvette should be 0. 4. On the board write the following chart. Test Tube 1 2 3

mL of Solution 2 4 6

mL of Water 8 6 4

Concentratio n (M) 0.08 0.16 0.24

4 5

8 10

2 0

0.32 0.40

5. Direct students to perform the dilutions as directed by the chart, mix with a stirring rod, then pipet into the cuvette and take an absorbance reading. 6. On graph paper or a spreadsheet program, direct students to graph their absorbance vs. concentration data. 7. Pass out either unknown A or unknown B to each group. 8. Direct students to determine the concentration of unknown A or B by comparing it by eye to the solutions they already have to see which it is between, and then find its absorbance. 9. For a fake absorbance value, show students how to use their graph to determine the concentration by using a ruler to mark the place where their absorbance value crosses the line, and then use the ruler to line up that point with the concentration. 10. Direct students to determine the concentration of their unknowns by this method. 11. Direct students to rinse the contents of their test tubes down the drain, and then put the test tubes in an ice cream bucket with water to soak. Clean the cuvettes with distilled water only. Student Questions: 1. What was the absorbance value of each of your test tubes? 2. Include the graph you made with this report. 3. What was the slope of the line of your absorbance graph? 4. What was the concentration of the unknown? 5. Beer’s law states that the absorbance is equal to the Molar Absorptivity * path length * concentration or A = ε * L * c. a. If the path length goes up, what will happen to absorbance? b. If the concentration goes down, what will happen to absorbance? c. If a substance, like a dye, had a much higher molar absorptivity than the copper sulfate, what would happen to the absorbance? Differentiations: 1. Remove the precalculated molarities. Direct students to use the dilution equation: M1*V1 = M2*V2 to determine the molarities in the chart.

2. Use food dyes. Food dyes have higher molar absorptivity and are more colorful than blue only. The colorimeter has a diode array of 430 nm [violet], 470 nm [blue], 565 nm [yellow], and 635 nm [red]. Some experimentation of which laser produces the greatest absorbance is necessary, but using these setting any concentration of any unknown should be decipherable. 3. Use consumer products instead of chemicals. Energy drinks, sodas, flavored water, and other drinks contain different amounts of dyes in them. By analyzing a sample in a cuvette and comparing it to a dilution, students can calculate the concentration of the dye. NGSS Alignment: MS-PS4-2; HS-PS4-4; CCSS Alignment: SL.8.5; MP.2; MP.4; 7.RP.A.2...