AP+Chem+Lab+5



** Determining the Concentration of a Solution: Beer’s Law ** Beer’s law, also called Beer-Lambert law, in spectroscopy, describes a relationship between the [|absorption] of [|radiant energy] by an absorbing medium and the concentration of that medium. Formulated by German mathematician and chemist August Beer in 1852, it states that the absorptive capacity of a dissolved substance is directly proportional to its concentration in a [|solution]. Beer's Law is often written in the form of this equation A = abc as a way of summarizing and quantifying the relationship between the absorbance, the nature of the absorbing chemical, the path length of the solution, and the concentration of the solution. It expresses the ideal situation in which these factors are truly proportional to the absorbance. much light will be absorbed by 1 cm of a 1 __M__ solution of this chemical. Its value depends on what the chemical is and also on what wavelength (or color) of light is being used. ||
 * Introduction: **
 * || The symbol "A" stands for the absorbance which you will measure with an instrument. ||
 * || The symbol "c" stands for concentration measured in molarity. ||
 * || The symbol "b" stands for the path length measured in centimeters. ||
 * || The symbol "a" is a proportionality factor called the molar absorptivity which is how

So, if all the light passes through a solution //without// any absorption, then absorbance is zero, and percent transmittance is 100%. If all the light is absorbed, then percent transmittance is zero, and absorption is infinite.

A **colorimeter** is a device used in [|colorimetry]. In scientific fields the word generally refers to the device that measures the [|absorbance] of particular [|wavelengths] of [|light] by a specific [|solution]. This device is most commonly used to determine the [|concentration] of a known [|solute] in a given solution by the application of the [|Beer-Lambert law], which states that the concentration of a solute is proportional to the absorbance. Different materials absorb different wavelengths of light. Therefore, the wavelength of maximum absorption by a material is one of the characteristic properties of that material. The %T can be related to the absorbance (A) by the equation below. A = 2.00 -[log (%T)]
 * Background: ** In the spectrum, light that ranges between 440 nm to 700 nm is the visible radiation. We also learn that the radiation results from electrons moving from one energy level to another. When an electron absorbs radiation (light), it is excited to a higher energy level. The absorption of light results in color in solutions of transition metal complexes.

If T = 85%, then Absorbance = 2 - log[85] = 0.071
 * // Beer's law //** states that the absorbance is directly proportional to the concentration of a solution. If you plot absorbance versus concentration, the resulting graph yields a straight line. The equation for the straight line (termed regression line) can be used to determine the concentration of an unknown solution once the %T has measured.
 * Concepts: **
 * Absorbance
 * Molar concentration
 * Beer’s Law


 * Experiment Overview: ** The primary objective of this experiment is to determine the concentration of an unknown copper (II) sulfate solution. The CuSO 4 solution used in this experiment has a blue color, so Colorimeter users will be instructed to use the red LED. A higher concentration of the colored solution absorbs more light (and transmits less) than a solution of l ower concentration. The Colorimeter monitors the light received as percent transmittance.

You will prepare eight copper (II) sulfate solutions of known concentration (standard solutions). Each solution is transferred to a small, rectangular cuvette that is placed into the Colorimeter. The amount of light that penetrates the solution and strikes the photocell is used to compute the absorbance of each solution. 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 is known as //Beer’s law//.

You will determine the concentration of an unknown CuSO 4 solution by measuring its absorbance. By locating the absorbance of the unknown on the vertical axis of the graph, the corresponding concentration can be found on the horizontal axis. The concentration of the unknown can also be found using the slope of the Beer’s law curve.


 * MATERIALS **
 * computer
 * 0.40 M copper (II) sulfate, CuSO4, solution
 * Vernier computer interface*
 * copper (II) sulfate, CuSO4
 * unknown solution
 * Vernier Colorimeter
 * pipet pump or pipet bulb
 * one cuvette
 * distilled water
 * five 20 × 150 mm test tubes test tube rack
 * two 10 mL pipets or graduated cylinders stirring rod
 * two 100 mL beakers tissues (preferably lint-free)


 * Procedure: **

1. Obtain and wear goggles. 2. Obtain 15 ml of 0.40 M CuSO4 solution and 15 ml of distilled water in separate beakers. 3. Label (on the lid) nine clean, dry, cuvettes 0–8. 4. Calculate the needed volumes of 0.40M CuSO4 and distilled water for each of the cuvettes to obtain the listed molarities. Each cuvette can hold 3 ml. 5. Use pipets to prepare the eight standard solutions according to the chart below. Thoroughly mix each solution.


 * ** Test Tube ** ||  ** 0.40 M CuSO4 (ml) **  ||  ** Distilled Water (ml) **  ||  ** Concentration **

** (M) ** ||
 * 1 ||   ||   ||  0.05  ||
 * 2 ||   ||   ||  0.10  ||
 * 3 ||   ||   ||  0.15  ||
 * 4 ||   ||   ||  0.20  ||
 * 5 ||   ||   ||  0.25  ||
 * 6 ||   ||   ||  0.30  ||
 * 7 ||   ||   ||  0.35  ||
 * 8 ||   ||   ||  0.40  ||

6. Prepare a //blank// by filling a cuvette 3/4 full with distilled water and mark this one 0 on the lid. To correctly use cuvettes, remember: • Wipe the outside of each cuvette with a lint-free tissue. • Handle cuvettes only by the top edge of the ribbed sides. • Dislodge any bubbles by gently tapping the cuvette on a hard surface. • Always position the cuvette so the light passes through the clear sides. 7. Connect a Colorimeter to Channel 1 of the Vernier Labpro. Connect the LabPro to the computer using the proper cable. 8. Start the Logger //Pro// program on your computer. Open the file “17 Colorimeter” from the //Advanced Chemistry with Vernier// folder. 9. Calibrate the Colorimeter. a. Place the blank in the cuvette slot of the Colorimeter and close the lid. b. Press the button on the Colorimeter to select the wavelength of 635 nm (Red). Press the CAL button until the red LED begins to flash and then release the CAL button. When the LED stops flashing, the calibration is complete. 10. You are now ready to collect absorbance-concentration data for the eight standard solutions. a. From the //Experiment// menu, choose **start collection**. b. Wipe the outside of the cuvette #1 with a tissue and place it in the colorimeter. Close the lid on the Colorimeter. c. **// When //** the absorbance readings stabilize, click KEEP from the //Experiment// menu, type the molarity of the solution in the cuvette in the edit box, and press the ENTER key. The data pair should now be plotted on the graph. d. Discard the cuvette contents as directed. e. Repeat the procedure for the rest of the cuvettes. f. When you have finished testing the standard solutions, click STOP COLLECTION from the //Experiment// menu. g. Examine the graph of absorbance //vs.// concentration. From the //Analyze// menu choose linear fit for the graph. A best-fit linear regression line will be shown for your data points. h. Print a graph showing the data and linear-regression equation for the standard solutions.
 * Note: ** Do not test the unknown solution until Step 10.

11. Write down the absorbance values, for each of the trials, in your data table. 12. Determine the absorbance value of the unknown CuSO 4 solution: a. Obtain about 3 mL of the //unknown// CuSO 4 in another clean, dry, cuvette. Record the number of the unknown in your data table. b. Rinse the cuvette twice with the unknown solution and fill it about 3/4 full. Wipe the outside of the cuvette, place it into the device. (Close the lid of the Colorimeter.) **Important**: The reading in the meter is live, so it is not necessary to click to read the absorbance value. c. Read the absorbance value displayed in the meter. When the displayed absorbance value stabilizes, record its value as Trial 9 in your data table. d. Repeat the above procedure for a second unknown solution. Record that information under Trial 10. d. Select Interpolate from the //Analyze// menu. Find the absorbance value that is closest to the absorbance reading you obtained in Step c above. Determine the concentration of your unknown CuSO 4 solution and record the concentration in your data table. e. Dispose of any of the remaining solutions as directed.


 * Data: **


 * ** Trial ** ||  ** Concentration (mol/L) **  ||  ** Absorbance **  ||
 * ** 1 ** ||   ||   ||
 * ** 2 ** ||   ||   ||
 * ** 3 ** ||   ||   ||
 * ** 4 ** ||   ||   ||
 * ** 5 ** ||   ||   ||
 * ** 6 ** ||   ||   ||
 * ** 7 ** ||   ||   ||
 * ** 8 ** ||   ||   ||
 * ** 9 ** || ** Unknown #_ ** ||   ||
 * ** 10 ** || ** Unknown #_ ** ||   ||

1. Print a graph showing the data and best fit line for the standard solutions. Show the absorbance and concentration values of the unknown solutions on your graph.
 * Post Lab Questions: **

2. Determine the concentration of the unknown CuSO4 solutions by using the equation found in your lab. Please be detailed and show your work. Use the slope from your graph for the molar absorptivity constant and the absorbance readings from the unknowns.

3. Obtain the accepted values for the two unknown solutions and calculate the percent error for both solutions.