12_Lab_113

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sFreezing Point Determination for Various Aqueous Solutions Introduction The purpose of this experiment is to determine how the concentration of solute in an aqueous solution impacts the freezing point of the solution. In chemistry, colligative properties exist which depend on the number of dissolved particles in a solution instead of the individual identity of the particles (French, et al. 66). Within this experiment, as the particles in a solution increase, the freezing point decreases. According to French, et al. “this can be represented by the equation: ΔT = i K f m where ΔT is the change in temperature from the original freezing point, i is the van’t Hoff factor, K f is the freezing point depression constant for the solvent, and m is the molality of the solution expressed in moles of solute per kilogram of solvent.” This equation can be used with the molality calculated in the experiment, along with the observed change in freezing point temperature to determine the freezing point depression constant. This value will be an integral part of the experiment to determine how varying concentrations of solute impact freezing points of solutions. In the experiment, freezing point depression constants will be calculated through various experiments that measure the freezing point of sugar solutions. A test tube of water will be prepared as a constant and an additional test tube of NaCl which will explore the impact the van’t Hoff factor has on the freezing point (French, et al. 66). To do this, dry ice must be used to create a temperature bath with temperatures low enough to test the freezing point of various solutions. Obtaining the freezing point in °C, will be determined graphically through extrapolation of flat temperature regions and initial drops (French, et al. 67). Procedure

Procedure and list of materials was inspired from (French, et al. 66-68). List of Materials  Temperature Probe  Beakers of Assorted Sizes  25ml Graduated Cylinder  Weigh boats  Analytical Balance  Tongs  Test tubes  Wash Bottle  250ml Erlenmeyer flask  Salt (NaCl)  Sucrose (table sugar, C 12 H 22 O 11 ) Measurenet 1. Connect temperature probe to workstation and power on 2. Press main menu and function key to select the temperature probe and then again to select “temperature vs time” experiment. 3. Calibrate the probe to room temperature 4. Set up axis limits for display purposes 5. Press “START/STOP” to begin or stop data collection Actual Experiment

1. Zero out the analytical balance with the weigh boat on it 2. Fill each weigh boat with 2g, 4g, 6g, and 8g of sugar 3. Prepare 4 sugar solutions with each of the masses of sugar and 25 ml of water. Ensure that 5ml of water is poured into a test tube at once with a maximum of 2g for each 5ml poured into the test tube while mixing at each stage to ensure solute is fully dissolved 4. Pour 5ml of each solution into a separate test tube 5. Weigh out .342g of NaCl and dissolve in a test tube with 25ml of water. 6. Pour 5ml of the salt solution into a separate test tube 7. Prepare a test tube with 5ml of water as a control 8. Weigh out 50g of rock salt and mix in a 250ml beaker with 200ml of water. Stir until dissolved 9. Use tongs to transport 150ml of dry ice pellets in a 400 ml beaker 10. Pour the saturated NaCl solution over the dry ice pellets and insert the temperature probe 11. Stir the solution until the ice bath is constant (-18°C to -25°C) 12. Insert the temperature probe into the test tube containing solely water and begin data collection 13. After 5 seconds, place tube in the prepared ice bath and continue to collect data until the graph has flatlined and the temperature has stopped changing or reversed, save as “001” 14. Repeat steps 12-13 for the 4 sugar solutions (save as “002-005”) and the salt solution (save as “006”) while drying and rinsing probe between each trial Discussion

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