Experiment 1 Using Freezing-PointDepression to Find Molecular Weight (note, this lab uses a small t for temperature, usually we use a large T for temperature and a small t for time…be aware as you read this) When a solute is dissolved in a solvent, the freezing temperature is lowered in proportion to the number of moles of solute added. This property, known as freezing-point depression, is a colligative property; that is, it depends on the ratio of solute and solvent particles, not on the nature of the substance itself. The equation that shows this relationship is Δt = Kf × m where Δt is the freezing point depression, Kf is the freezing point depression constant for a particular solvent (3.9°C•kg/mol for lauric acid in this experiment1), and m is the molality of the solution (in mol solute/kg solvent). OBJECTIVES In this experiment, you will Determine the freezing temperature of the pure solvent, lauric acid. Determine the freezing temperature of a mixture of lauric acid and benzoic acid. · Calculate the freezing point depression of the mixture. · Calculate the molecular weight of benzoic acid. · · Figure 1 MATERIALS Vernier computer interface computer lauric acid-benzoic acid mixture hot water bath 111 “The Computer-Based Laboratory,” Journal of Chemical Education: Software, 1988, Vol.1A, No. 2, p. 73. Advanced Chemistry with Vernier 4-1 Temperature Probe ring stand 400 mL beaker lauric acid, CH3(CH2)10COOH two 18 × 150 mm test tubes utility clamp paper towel or tissue PROCEDURE 1. Obtain and wear goggles. 2. Connect a Temperature Probe to Channel 1 of the Vernier computer interface. Connect the interface to the computer with the proper cable. 3. Start the Logger Pro program on your computer. Open the file “04 Freezing Point” from the Advanced Chemistry with Vernier folder. Part I Determine the Freezing Temperature of Pure Lauric Acid 4. Add about 300 mL of tap water with a temperature of 20-25°C to a 400 mL beaker. Place the beaker on the base of the ring stand. 5. Use a utility clamp to obtain a test tube containing hot, melted lauric acid from your instructor. Fasten the utility clamp at the top of the test tube. CAUTION: Be careful not to spill the hot lauric acid on yourself and do not touch the bottom of the test tube. 6. Insert the Temperature Probe into the hot lauric acid. Fasten the utility clamp to the ring stand so the test tube is above the water bath. Click to begin the data collection. 7. Lower the test tube into the water bath. Make sure the water level outside the test tube is higher than the lauric acid level inside the test tube, as shown in Figure 1. 8. With a very slight up-and-down motion of the Temperature Probe, continuously stir the lauric acid for the ten-minute duration of the experiment. 9. When the data collection is complete, use a hot water bath to melt the lauric acid enough to safely remove the Temperature Probe. Carefully wipe any excess lauric acid liquid from the probe with a paper towel or tissue. 10. The freezing temperature can be determined by finding the mean temperature in the portion of the graph with nearly constant temperature. a. Move the mouse pointer to the beginning of the graph’s flat part. Press the mouse button and hold it down as you drag across the flat part of the curve, selecting only the points in the plateau. b. Click the Statistics button, . c. The mean temperature value for the selected data is listed in the statistics box on the graph. Record this value as the freezing temperature of pure lauric acid. d. Click on the upper-left corner of the statistics box to remove it from the graph. Part II Determine the Freezing Temperature of a Solution of Benzoic Acid and Lauric Acid 4-2 Advanced Chemistry with Vernier 11. Using Freezing Point Depression to Find Molecular Weight Prepare the computer for data collection. e. From the Experiment menu, choose Store Latest Run. This stores the data so it can be used later. f. To hide the curve of your first data trial, click the Temperature y-axis label of the graph and choose More…. from the dropdown list. Uncheck the Run 1 Temperature box and click . 12. Obtain a test tube containing a melted solution with ~1 g of benzoic acid dissolved in ~ 8 g of lauric acid. Record the precise masses of benzoic acid and lauric acid as indicated on the label of the test tube. Repeat Steps 4–9. …..we will discuss this bad picture…. 13. The freezing temperature of the benzoic acid-lauric acid solution can be determined by finding the temperature at which the mixture initially started to freeze. Unlike pure lauric acid, the mixture results in a gradual linear decrease in temperature during freezing. Follow the steps below to analyze the graph. **It is important that you determine this freezing point graphically and NOT take a printout of the curve and determine it at home. g. Click and drag the mouse to highlight the initial part of the cooling curve where the temperature decreases rapidly (before freezing occurred). h. Click on the Linear Regression button, . i. Now click and drag the mouse over the next linear region of the curve (the gently sloping section of the curve where freezing took place). j. Click again. The graph should now have two regression lines displayed. k. Choose Interpolate from the Analyze menu. Move the cursor to the point of intersection of the two lines; you will know if the cursor is over the intersection when the temperature readings displayed in the interpolate box are the same. This is the freezing point of the benzoic acid-lauric acid mixture. l. Record the freezing point in your data table. 14. Print a graph showing both trials. DATA TABLE Mass of lauric acid (g) Mass of benzoic acid (g) Advanced Chemistry with Vernier 4-3 Freezing temperature of pure lauric acid (°C) Freezing point of the benzoic acid-lauric acid mixture (°C) Data Analysis and Questions: 1. Calculate molality (m), in mol/kg, using the formula Δt = Kf × m. The Kf value for lauric acid is 3.9°C•kg/mol. 2. Calculate moles of benzoic acid solute, using the molality and the mass (in kg) of lauric acid solvent. 3. Calculate the experimental molecular weight of benzoic acid, in g/mol from the mass and the number of mol of benzoic acid. 4. Determine the accepted (theoretical) molecular weight of benzoic acid from its formula, C6H5COOH. 5. Calculate the percent error (% error = 100x|accepted – experimental| / accepted) between the experimental and accepted values. 6. If you fail to stir the material in the test tube, would you expect the reported freezing point to be too high or too low? Why? 7. If you spill a little of the benzoic acid as you mix it with the lauric acid will the reported freezing point be too high or too low? Why? 8. If you spill a little of the benzoic acid as you mix it with the lauric acid will the molecular weight you calculate for the benzoic acid be too high or too low? Why? 9. If a small amount of the benzoic acid fails to dissolve in the lauric acid, will the molecular weight you calculate for the benzoic acid be too high or too low? Why? 10. List all errors you know you made in this experiment and explain how each influences the result of the experiment. These errors might include errors in temp measurement, mass measurements, errors associated with the stirring and mechanical activities in the lab. 11. How does the concentration of a solution impact the boiling and freezing points for that solution? Explain why this happens using a physical description. 4-4 Advanced Chemistry with Vernier Using Freezing Point Depression to Find Molecular Weight 12. How could this method be used to understand the nature of a contaminant in a solution? Could it be used to determine the chemical formula for the contaminant? Why or why not? Conclusion Write a short conclusion for this lab. Briefly discuss your results (MW and % error should be given) and what you learned Advanced Chemistry with Vernier 4-5
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