Chemistry 113, Laboratory 12 - Freezing Point Depression Of Water PDF

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Professor: April French
Lab 12: Determining Freezing Point Depression of Water...

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Jessica Flesch CHE 113, 002 Yuting Zhang Ami Patel 17 February, 2017

Lab 12: Determining Freezing Point Depression of Water Introduction: The purpose of this lab is to determine the freezing point depression of water, observe the relationship between freezing point and the sugar concentration, and to compare the freezing points of sugar and salt solutions of the same concentration (French, et al. 69). Colligative properties, which are properties that depend on how many dissolved particles are in a solution, will be important in this experiment. Two colligative properties used in this lab are boiling point and freezing point. When the concentration of particles in a solution is increased, the freezing point will decrease while the boiling point will increase (French, et al. 70). This means that there is a direct relationship between the number of particles in solution and the deviation from the normal boiling and freezing points. The equation below is used to represent freezing point depression: ΔT = iKfm In the equation, ΔT represents the change in temperature from the original freezing point, i is the van’t Hoff factor, Kf is the freezing point depression constant for the solvent, and m is the solution's molality (French, et al. 70). To find molality, divide the number of moles of the solute by kg of solvent. The solvent's mass is not temperature dependent, so this experiment uses molality instead of molarity. The following equation is used to calculate the molality of a solution: m= (mol of solute)/(kg solvent) i, the van't Hoff factor, will change depending on the solution. Since NaCl is an electrolyte, its i value will be 2. The sugar solution is a nonelectrolyte, so its i value is only 1. The van't Hoff factor will have a direct relationship with the freezing point depression. The freezing point depression constant of water is what needs to be found. This will be determined by gathering data on the freezing point of several sugar and water solutions. The sugar solutions will then be compared to a sodium chloride solution on account of the effect of the van't Hoff factor.

Methods: Materials (French, et al. 72)

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Salt Sucrose 250mL beaker MeasureNet Temperature probe Other glassware and equipment, as needed 6" test tubes Wash bottle Ring stand Buret Clamp

Procedure: 1. Mix 50-60g of NaCl in 200mL of water. 2. Put 150mL of dry ice pellets into a 400mL beaker. Use tongs to retrieve the dry ice. 3. Pour the NaCl solution over the dry ice in the beaker until the temperature becomes constant around –18 to 25°C. 4. Put 25mL of distilled water in 6 test tubes. 5. Set aside one test tube to just remain pure water. 6. In the second test tube, add approximately 1g of sugar. 7. In the third test tube, add approximately 2g of sugar. 8. In the fourth test tube, add approximately 3g of sugar. 9. In the fifth test tube, add approximately 4g of sugar. 10. Record exact masses of solutes and solvents in each test tube. 11. In the sixth test tube, add enough salt so that the concentration of test tube 5 is equal to molality of the first sugar solution. 12. Calibrate the temperature probe. 13. Rinse the temperature probe between each use. 14. Press "Power" > "Main menu." Select the temperature vs. Time option. 15. Press "Display" and use the "Start/Stop" button to begin and end the data collection. 16. Hold the temperature probe in the solution for 100-200 seconds. 17. Save the first graph as "001" and the second as "002" and follow this pattern for all six trials. 18. Check with your TA after each trial to make sure that the graph is correct before moving on to the next trial. 19. When the lab is complete, clean and wash all of your supplies. Dispose of all substances used properly. Discussion: The purpose of this lab is to determine the freezing point depression of water, observe the relationship between freezing point and the sugar concentration, and to compare the freezing points of sugar and salt solutions of the same concentration (French, et al. 69). This will be done using a salt solution and sugar solutions of varying concentrations.

The average calculated value for Kf was 12.76597. The average value for Kf based on the graph was 5.8648. The percent error for the calculated value was 586.3424%. The percent error for the value based on the graph was 215.3118%. The theoretical freezing point of the salt solution was found to be –3.3284°C. The results support the original hypothesis because the freezing point depression was able to be determined based on the data gathered in the lab. It was observed that as the molality of the sugar solution increased, the freezing point was lowered. The change in freezing point for the NaCl solution is greater than that of the sugar solution because the i values are different. NaCl is an electrolyte, so it's i value is 2. For sugar, the i value is 1. When making the calculations, the change in freezing point for the NaCl solution is greater because it has a larger i value. There were some potential sources of error in this experiment. The first error could have occurred when the sugar was being gathered for the solutions. After the sugar was weighed, it had to be transferred to the test tube. During this process, some of the sugar may have been lost. This would have made the weight of the solute less than the recorded value, which would have made the actual concentration of the solution be less than the calculated concentration. This error could have been avoided by being more careful when transferring the solute to the test tubes. A second source of error occurred when the solutions were being mixed. The sugar did not completely dissolve in the solvent when it was first added. To fix this, the solution had to be mixed by pouring the solution back and forth between the test tube and a clean beaker. During this process, some of the solute may have been left in the beaker used for mixing. The result of this error would be that the actual weight and concentration of the solute would be less than the calculated values. This could have been avoided by making sure that the beaker used for mixing was completely empty when the mixing process was complete. A third source of error occurred when the same temperature probe was used for each trial. The probe was placed in multiple test tubes, which means that it was exposed to both the salt and the sugar solutions. Although the probe was rinsed between each trial, there could have been residual solute left on it from certain solutions. This residue would then be transferred to the next solution that was measured. The result of this error is that salt may have would up in the sugar solution or vice versa. Another possible result is that there could be extra solute in certain solutions, making the concentration higher than the calculated value. To avoid this error, the temperature probe could have been more thorough cleaned and dried between each trial.

Conclusion: In this experiment, I learned how to determine the freezing point depression of water. I also learned how to use the graphs generated during lab to find Kf and to compare the freezing points of solutions with different solutes at the same concentration. The skills I have learned in this lab have several real life applications. Examples of the importance of freezing point depression can be observed by the use of antifreeze and salt on the roads. Both of these products work to lower the temperature at which different substances will

freeze to keep them at a liquid state for as long as possible. Another application of freezing point depression in everyday life is making ice cream. Salt is added to the solution used to make ice cream in order to lower its freezing point and allow it to stay liquid long enough to be properly mixed. Many chemists use freezing point depression as a tool in cryoscopy, which is used to measure the degree of disassociation of a solute.

Works Cited: French, April, Allison Soult, Stephen Testa, Pauline Stratman, M. Meral Savas, Francois Botha, Caroly Brock, Charles Griffith, Darla Hood, Robert Kiser, Penny O'Connor, William Plucknett, Diane Vance, and William Wagner. General Chemistry Laboratory Manual. “Experiment 12: Freezing Point Depression”. “Plymouth, MI: Hayden-McNeil Publishing, 2016. (69-72). 12 February. 2017. French, April, Allison Soult, Stephen Testa, Pauline Stratman, M. Meral Savas, Francois Botha, Caroly Brock, Charles Griffith, Darla Hood, Robert Kiser, Penny O'Connor, William Plucknett, Diane Vance, and William Wagner. General Chemistry Laboratory Manual. “Chapter 4: Using Measurenet and Probes”. “Plymouth, MI: Hayden-McNeil Publishing, 2016. (37-46). 12 February. 2017....