Determining the Concentration



Determining the Concentration

of a Solution: Beer’s Law

(adapted from Advanced Chemistry with Vernier, Lab 17)

The primary objective of this experiment is to determine the concentration of an unknown copper (II)sulfate, CuSO4, solution. You will use a Vernier Spectrometer or a Vernier SpectroVis to measure the concentration of each solution. You will first measure the absorbance of a standard solution over the visible light spectrum and select the wavelength of maximum absorbance (λ max). A higher concentration of the solution absorbs more light (and transmits less) than a solution of lower concentration.

You will prepare five copper (II) sulfate solutions of known concentration (standard solutions) and measure the absorbance of the standard solutions at the λ max. The graph of absorbance vs. concentration for the standard solutions will describe a direct relationship, known as Beer’s law.

Finally, you will determine the concentration of an unknown CuSO4 solution by measuring its absorbance with a spectrometer and using the best-fit line equation of the Beer’s law curve to calculate the unknown’s concentration.

OBJECTIVES

In this experiment, you will

• Measure and analyze the visible light absorbance spectrum of a standard copper (II) sulfate solution to determine the maximum wavelength of absorbance.

• Prepare and test the absorbance of five standard copper (II) sulfate solutions.

• Calculate a standard curve from the test results of the standard solutions.

• Test the absorbance of a copper (II) sulfate solution of unknown molar concentration.

MATERIALS

|Vernier SpectroVis |0.40 M copper (II) sulfate, CuSO4, solution |

|computer |copper (II) sulfate, CuSO4, unknown solution |

|Vernier Logger Pro 3 software |distilled water |

|pipet pump or pipet bulb |test tube rack |

|one cuvette |stirring rod |

|two 10 mL pipets |tissues (KimWipes) |

|five 20 × 150 mm test tubes |two 100 mL beakers |

PROCEDURE

1. Prepare a data table in your lab notebook (like the one at the end of this experiment).

2. Obtain and wear goggles.

3. Obtain ~40 mL of 0.40 M CuSO4 solution and distilled water in separate beakers.

4. Label five clean, dry, test tubes 1–4. Use pipets to prepare the standard solutions according to the chart below (one of the standards is the stock 0.40 M CuSO4 solution). Thoroughly mix each solution with a stirring rod. Clean and dry the stirring rod between uses.

|Test Tube |0.40 M CuSO4 |Distilled H2O |Concentration |

|number |(mL) |(mL) |(M) |

|1 |2 |8 |0.080 |

|2 |4 |6 |0.16 |

|3 |6 |4 |0.24 |

|4 |8 |2 |0.32 |

5. Use a USB cable to connect a Vernier Spectrometer or a Vernier SpectroVis to a computer.

6. Start the Logger Pro 3 (version 3.6 or newer) program on your computer.

7. Calibrate the spectrometer.

a. Prepare a blank by filling an empty cuvette ¾ full with distilled water.

a. Open the Experiment menu and select Calibrate → (Spectrometer). The following message appears in the Calibrate dialog box: “Waiting … seconds for the device to warm up.” After the allotted time, the message changes to: “Warmup complete.”

b. Place the blank in the spectrometer; make sure to align the cuvette so that the clear sides are facing the light source of the spectrometer. Click “Finish Calibration”, and then click [pic].

8. Determine the ( max for CuSO4 (aq) and set up the data collection mode.

a. Empty the blank cuvette and rinse it twice with small amounts of the 0.32 M CuSO4 solution in Test Tube 4. Fill the cuvette ¾ full with the 0.32 M CuSO4 solution and place it in the spectrometer.

c. Click [pic]. A full spectrum graph of the CuSO4 solution will be displayed. Note that one area of the graph contains a peak absorbance (( max). Click [pic] to complete the analysis.

d. To save your graph of absorbance vs. wavelength, select Store Latest Run from the Experiment menu.

e. Click the Configure Spectrometer icon, [pic], on the toolbar. A dialog box will appear.

f. Select Abs vs. Concentration under Set Collection Mode. The ( max will be automatically selected. Click [pic] to proceed.

9. Collect absorbance-concentration data for the five standard solutions.

a. Leave the cuvette in the spectrometer. Click [pic]. When the absorbance reading stabilizes, click [pic]. Enter “0.32” (the solution concentration). Click [pic].

g. Discard the cuvette contents as directed. Using the solution in Test Tube 3, rinse and fill the cuvette ¾ full. Wipe the cuvette and place it in the spectrometer. When the absorbance reading stabilizes, click [pic]. Enter “0.24” and click [pic].

h. Repeat Step 8b for the remaining test tubes of the standard CuSO4 solution. When you have finished testing the standard solutions, click [pic].

10. To determine the best-fit line equation for the CuSO4 standard solutions, click the linear fit button, [pic], on the toolbar. Write down the equation for the standard solutions in your data table or lab book.

11. Determine the concentration of the unknown CuSO4 solution.

a. Obtain about 5 mL of the unknown CuSO4 solution in test tube 5..

i. Rinse the cuvette twice with the unknown solution and fill it about ¾ full. Wipe the outside of the cuvette and place it into the spectrometer.

j. Select Interpolation Calculator, from the Analyze menu. A dialog box will appear that displays the concentration of your unknown at the measured absorbance.

k. Click [pic]. Write down the absorbance and the concentration of the unknown in your data table or lab book.

l. Dispose of any of the remaining solutions as directed.

DATA TABLE

|Test Tube |Concentration (mol/L) |Absorbance |

|4 |0.32 | |

|3 |0.24 | |

|2 |0.16 | |

|1 |0.04 | |

|5 |Unknown | |

DATA ANALYSIS

Your lab report should contain the following information:

• a graph showing the data and linear-regression equation for the standard solutions

• the molar concentration of your unknown CuSO4 solution

• an explanation of your calculation of the unknown’s concentration

POST-LAB EXERCISE:

A student is instructed to determine the concentration of a solution of CoCl2 based on the absorption of light (spectrometric/colorimetric method). The student is provided with a 0.10 M solution of CoCl2 with which to prepare standard solutions with concentrations of 0.020 M, 0.040 M, 0.060 M, and 0.080 M.

a. Describe the procedure for diluting the 0.10 M solution to a concentration of

0.020 M using distilled water, a 100 mL volumetric flask, and a pipette or buret. Include specific amounts where appropriate.

The student takes the 0.10 M solution and determines the percent transmittance and the absorbance at various wavelengths. The two graphs below represent the data.

[pic]

b. Identify the optimum wavelength for the analysis.

The student measures the absorbance of the 0.020 M, 0.040 M, 0.060 M, 0.080 M, and 0.10 M solutions. The data are plotted below.

[pic]

c. The absorbance of the unknown solution is 0.275. What is the concentration of the

solution?

d. Beer’s Law is an expression that includes three factors that determine the amount

of light that passes through a solution. Identify two of these factors.

e. The student handles the sample container (e.g., test tube or cuvette) that holds the

unknown solution and leaves fingerprints in the path of the light beam. How will

this affect the calculated concentration of the unknown? Explain your answer.

m. Why is this method of determining the concentration of CoCl2 solution appropriate, whereas using the same method for measuring the concentration of NaCl solution would not be appropriate?

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