Molarity Activity

The following two tasks are to be completed today. Items in bold must be included in your science notebook to get credit for this activity.

1) Your first group task is to prepare 50-mL of a 1.0-M solution of sucrose, C₁₂H₂₂O₁₁. Each of you should write down any pertinent calculations and record a written description of how you prepared the solution. You are to use a graduated cylinder for measurement and a beaker to mix the sugar and water. You will find the sucrose near the balances on the side counter near the shower in the room.
Once you have prepared the solution you are to bring the beaker containing the solution to your instructor who will determine if you have done the task properly. Wait for your instructor's permission before you proceed.

2) Your second group task is to determine the concentration of an unknown solution. You will use a Colorimeter (a side view is shown in Figure 1) that measures the concentrations of solutions. In this experiment, red light from the LED light source will pass through the solution and strike a 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.

Figure 1

You will prepare 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.

• Make calculations, explain in writing how to make solutions of 0.08 M, 0.16M, 0.24, 0.32M,  and 0.40M from the 0.40M solution given and make your solutions in five different clean test tubes using measuring devices in cupboard. 

• Connect a Colorimeter to Channel 1 of the Vernier computer interface. Connect the interface to the computer using the proper cable.

• Start the Logger Pro program on your computer. Open the file “17 Colorimeter” from the Advanced Chemistry with Vernier folder.

• Calibrate the Colorimeter by following the following steps.

• 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.

• If your Colorimeter has a CAL button, set the wavelength on the Colorimeter to 635 nm, press the CAL button, and proceed directly to Step 7. If your Colorimeter does not have a CAL button, continue with this step to calibrate your Colorimeter.

• Choose Calibrate } CH1: Colorimeter from the Experiment menu, then click .

• Turn the wavelength knob on the Colorimeter to the “0% T” position.

• Type 0 in the edit box.

• When the displayed voltage reading for Reading 1 stabilizes, click .

• Turn the knob of the Colorimeter to the Red LED position (635 nm).

•  Type 100 in the edit box.

• When the voltage reading for Reading 2 stabilizes, click , then click .

• You are now ready to collect absorbance-concentration data for the five standard solutions so Click ..

• 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.

• When the absorbance readings stabilize, click , type “0.080” in the edit box, and press the ENTER key. The data pair should now be plotted on the graph.

• Discard the cuvette contents as directed. Using the solution in Test Tube 2, rinse and fill the cuvette ¾ full. Wipe the outside, place it in the Colorimeter, and close the lid. When the absorbance readings stabilize, click , type “0.16” in the edit box, and press the ENTER key.

• Repeat the procedure for Test Tubes 3 and 4. Trial 5 is the original 0.40 M solution. Note: Do not test the unknown solution until told to do so.

• When you have finished testing the standard solutions, click .

• Examine the graph of absorbance vs. concentration. Click the Linear Regression button, . A best-fit linear regression line will be shown for your five data points.

• Record the absorbance values, for each of the five trials, in your data table copied into your notebook from below. Print a graph of your data for each member of your group to put into your notebook.

• Determine the absorbance value of the unknown solution by obtaining about 5 mL of the unknown in another clean, dry, test tube. 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.

• Read the absorbance value displayed in the meter. (Important: The reading in the meter is live, so it is not necessary to click  to read the absorbance value.) When the displayed absorbance value stabilizes, record its value as Trial 6 in your data table.

• Determine the concentration of the unknown solution. Explain how you made this determination.

• Dispose of any of the remaining solutions in the sink.