Lauric Acid Freezing Point Lab Writeup PDF

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Rachel Tammone CHM113 Section 7R Heather Pedziwiatr 4/5/14 Freezing Point Depression: Lauric Acid Purpose: The purpose of this lab is to determine the freezing point depression constant (Kf) for lauric acid by using Lab Pro. Kf can be considered because Lauric acid is a non-electrolyte. It is important to learn about freezing points because in the process of freezing, impurities interfere with normal molecular interactions. Impurities lower the temperature needed for a solid to solidify. This becomes useful to understand and know in regions of the world where snow is predominant. Putting salt down on the roads prevents icing by lowering the freezing point of water. Theory: In order to perform this experiment successfully, pure lauric acid, .75g BA/ 8g LA, and 1.50g BA/ 8g LA will be monitored with a temperature probe in order to determine the freezing point depression constant. The samples must be heated until they are fully liquefied and then placed in a beaker of cold water so they solidify. As the samples solidify the temperate probe will keep track of the temperature, so the freezing point and change in temperature can be found. The formula ΔT=Kf m is used to find the freezing point depression constant after figuring out the change in temperature and the molality of the samples Molality equals the moles of solute divided by the mass of the solvent. Averaging the freezing point depression constant from the two impure samples allows one to find the average Kf of lauric acid. Procedure: 1. Attach the temperature probe to CH1 on the Lab Pro interface, & connect the interface to the computer. Start the Logger Pro software & open the appropriate files so the title of the graph reads “freezing point of lauric acid” 2. Place a 400 mL beaker filled with water on a hotplate, but do not turn the hotplate up past three. 3. Obtain a test-tube containing pure lauric acid and place it in the beaker so it melts. When it is fully melted, remove it from the water bath. 4. Put the temperature probe into the test tube and being stirring the molten lauric acid. Wait 5 seconds then hit collect on the Lab Pro screen on the computer. 5. Place the test tube with the temperature probe in it in a beaker filled with cold/room temperature water. 6. Constantly use the temperature probe to stir the lauric acid as it cools. Once the flat area of the cooling curve is reached, stirring is not necessary. 7. Add a few ice chips to the beaker containing the test tube to aid in freezing. Once the temperature drops below 30° Celsius, data no longer needs t be recorded. 8. Highlight the flat portion of the curve and hit line of best fit. The y-intercept of the line is the freezing point of the lauric acid. 9. Make sure to save the data & use an appropriate file name when doing so. Also hit “Store latest run” to get a blank graph for the upcoming experiment. 10. Take the temperature probe out of the now frozen lauric acid, re-melting it if necessary, and return the test tube. 11. Repeat steps 1 through 10 using the .75g BA/8g LA & the 1.50g BA/8g LA

12. Determine the molality of both impure samples of lauric acid. Use the molality & change in temperature to determine Kf. Data/Calculations: Mp of pure Lauric Acid: 43.22° C & 43.22° C AVG: 43.22° C -The freezing point (43.22°C) was found by highlighting the flat part of the curve on Lab Pro & clicking line of best fit. It is the y-intercept. Benzoic Acid: molecular weight = 122.12 g/mol - The molecular weight was found by adding up all the weights of each element in the acid. The weight of an element was added however many times that element is found in the acid. .750g BA/8g LA: m = .768 ΔT = 2.49°C Kf = 3.24 - Molality (m) was found by dividing the moles of BA by the weight of LA (in kg) o .0061/.008 = .768 - Change in temp was found by subtracting the freezing point of pure lauric acid by the freezing point of the .750g BA sample o 43.22-40.73 = 2.49 - Kf was found by using the formula ΔT=Kf m o 2.49 = (Kf)(.768) 1.50g BA/8g LA: m = 1.54 ΔT = 5.53°C Kf = 3.54 - Molality (m) was found by dividing the moles of BA by the weight of LA (in kg) o .0123/.008 = 1.54 - Change in temp was found by subtracting the freezing point of pure lauric acid by the freezing point of the 1.50g BA sample o 43.22-37.69 = 5.53 - Kf was found by using the formula ΔT=Kf m o 5.53 = (Kf)(1.54) Average Kf: 3.42 - Average Kf was found by averaging the Kf s of the .750g BA sample & the 1.50g BA sample o 3.24+3.59 = 6.83 6.83/2 = 3.42

Discussion/Error Analysis: Error could have happened throughout this experiment when using the Lab Pro system and software. It often can be unreliable and a little glitchy. If the equipment was not working properly, the freezing points found would be inaccurate which would then make all other calculations that use the freezing point inaccurate. However, because the freezing point found for pure lauric acid the first time was 43.22°C, and that is the actual freezing point, I believe the equipment was working well. Another source of error could have came from the samples themselves and their molalities and molarities. Both rely on the weight of the solvent, solute, or solution. We were told the weights in the lab manual, however that does not mean the samples we used actually contained the correct amount. People in previous lab sections who used the same materials could have spilled some of the lauric acid out, changing the actual weight of the sample. Conclusion: In the lab, the freezing point depression constant was found for lauric acid using the temperature probe/Lab Pro software. Manual data calculations using the formulas ΔT=Kf m and molality equals the moles of solute divided by the mass of the solvent was also needed to properly analyze the data collected from the temperature probe. Overall, the lab carried out its purpose in teaching how to calculate the freezing point depression constant....