At-Home Colligative Properties: Chemistry Experiment Freezing Point Determination Objectives Colligative properties of solutions depend on the quantity of solute dissolved in the solvent rather than the identity of the solute. The phenomenon of freezing point lowering will be examined quantitatively as an example of a colligative property in this at-home experiment. Introduction When a solute is dissolved in a solvent, the properties of the solvent are changed by the presence of the solute. The magnitude of the change generally is proportional to the amount of solute added. Some properties of the solvent are changed only by the number of solute particles present, without regard to the particular chemical nature of the solute. Such properties are called colligative properties of the solution. Colligative properties include the changes in vapour pressure, boiling point, freezing point and the phenomenon of osmotic pressure. If a non-volatile solute is added to a volatile solvent (such as water), the amount of solvent molecules that can escape from the surface of the liquid at a given temperature is lowered compared to the situation where only pure solvent is present. The vapour pressure above such a solution will thus be lower than the vapour pressure above a sample of the pure solvent under the same conditions. Molecules of the non-volatile solute physically block the surface of the solvent, thereby preventing as many molecules from evaporating. This results in an increase in the boiling temperature of the solution as well as a decrease in the freezing point. In this at-home experiment, the freezing points will be measured for: 1. water 2. a solution of sucrose and water at 3 different concentrations. The freezing point of the sucrose solutions will be used to determine its molar mass. The decrease in the freezing point, T, when a non-volatile, nonionizing solute is dissolved in a solvent is proportional to the molal concentration, m, of the solution, ∆푇 = 퐾푓 ∙ 푚 (1) Here, ∆푇 = 푇° − 푇 (2) T° is the freezing temperature of pure water, and T is the freezing temperature of the sucrose- water solution. The freezing point depression constant, Kf, is a constant for a given solvent and establishes the number of degrees that the freezing point will be lowered when one mole of solute is dissolved in one kilogram of solvent. The molal concentration is defined as 1 푛푢푚푏푒푟 표푓 푚표푙푒푠 푠표푙푢푡푒 (3) 푚표푙푎푙푖푡푦, 푚 = 푘푖푙표푔푟푎푚푠 푠표푙푣푒푛푡 푚푎푠푠 표푓 푠표푙푢푡푒 (4) 푚표푙푎푟 푚푎푠푠, 푀푀 = 푛푢푚푏푒푟 표푓 푚표푙푒푠 표푓 푠표푙푢푡푒 In Parts A and B, the solutions to be used in the experiment as well as the ice/salt bath are prepared. In Part C, the freezing points of pure water and of the sucrose-water solutions are measured respectively; the decrease in the freezing temperature, T, between each of the two solutions can then be obtained. From the T of the solutions used, and the known Kf for water of 1.86°Ckgmol-1, the molality of the solution can be calculated using 1. The known masses of the solute and solvent, as well as the measured molality, will then be used to obtain the molar mass of sucrose using 3 and 4. After the experiment, you are encouraged to employ the same principles of freezing point depression to make your own ice cream at home. Materials Thermometer 12 Tablespoons of white granulated 25 mL or 50 mL syringe sugar (sucrose) 3 containers to store dissolved 50 mL polypropylene tube sucrose solutions (A, B and C). Each Water container should hold a minimum of Ice (crushed or cubed) 100 mL Salt (table salt, or kosher salt / Large container that can hold coarse salt / de-icing salt), enough to approximately 400-600 mL to be achieve a 2-3 cm layer in chosen used for the ice/salt bath container Tablespoon / measuring spoon Procedure Part A - Preparation of the Sucrose Solutions 1. To one of the containers, add 2 tablespoons of sucrose and 50 mL of water using the syringe. Stir the solution, cover and leave overnight (or a minimum of 8 hours) to completely dissolve the sucrose. Label the container as Solution A. 2. To a second container, add 4 tablespoons of sucrose and 50 mL of water using the syringe. Stir the solution, cover and leave overnight (or a minimum of 8 hours) to completely dissolve the sucrose. Label the container as Solution B. 3. To a third container, add 6 tablespoons of sucrose and 50 mL of water using the syringe. Stir the solution, cover and leave overnight (or a minimum of 8 hours) to completely dissolve the sucrose. Label the container as Solution C. 2 Figure 1: Dissolved Sucrose Solutions A, B and C Part B - Preparation of the Ice/Salt Bath 1. Half fill the large container with crushed ice, pour a 15 mm to a 25 mm layer of salt on top of the ice, and using a spoon, stir until the ice and salt are thoroughly mixed. 2. Immerse the thermometer in the ice/salt bath, allow a couple minutes for it to stabilize, and record the temperature. (Table 2) a) b) Figure 2: a) Ice/Salt Bath, b) Temperature of Ice/Salt Bath 3. If at any point during the experiment more ice/salt is required so that it is above the liquid line in your polypropylene sample tube, add it carefully so that it does not mix with the sample being tested. 3 Part C - Determination of the Freezing Point of Water and Sucrose Solutions (A, B and C) 1. Half fill a polypropylene tube with 25 mL of water and carefully press it down into the ice/salt bath until the water level is completely below the surface of the ice/salt bath. Start a stopwatch or timer after it is immersed. (Figure 3 b) a) b) c) Figure 3: a) Polypropylene tube, b) measuring solution's freezing point, c) frozen sucrose solution 2. Gently stir the water with the thermometer continuously and record the temperature to one value after the decimal every 30 seconds until it stabilizes for a minimum of 5 constant readings. At this point, ice crystals will have begun to form on the sides of the polypropylene tube. It may be necessary to remove the tube from the ice/salt bath periodically to check the status. Ice crystals tend to form first near the bottom of the test tube. (Figure 3 c) 3. Once the data has been recorded, stop the timer, clean the tube with warm water to dissolve the frozen water and remove the contents inside the tube. Dry the tube before reusing in the next step. 4. Repeat the procedure (Part C steps 1-3) with each of the 3 sucrose solutions (A, B and C) to be tested in place of the water and record the results for each run. 4 Calculations and Data Analysis 1. Plot the collected values of Temperature (°C) vs Time (seconds) in 4 separate graphs in Excel (one for each solution to be measured). Label the title of the graphs and axes appropriately. 2. Look for the most linear portion of the graph after the dip and before the values begin to decrease again. Calculate the average temperature of those values. The resulting value is the freezing point (f.p.) of the sample. See circled values to represent the linear portion of the curve (Figure 4). Figure 4: Sample data for the freezing point of water 3. Repeat the determination of the freezing point for solutions A, B, and C. Record the values of the freezing points (Table 2). 4. Calculate the T using the freezing points for each sucrose solution and water (2). 5. Calculate the molality of the sucrose solutions using the calculated T and the Kf of water 1.86°Ckgmol-1 (1). 6. Using the calculated molality, determine the number of moles of sucrose used (3). Convert the volume of water used to prepare the solutions into mass of water used. Assume the density of water is 1.00 g/mL. 7. Based on your experimental data, calculate the molar mass of sucrose (Equation 4). You will need to convert the volume of sucrose used to prepare the solutions into mass of sucrose used. The mass of 1 tablespoon of sucrose (sugar) is 12.782g. https://www.convertunits.com/from/gram+[sugar]/to/tablespoon 8. Calculate the percent error using the theoretical molar mass of sucrose. 9. Calculate the average molar mass and percent error and complete the data analysis table (Table 2) 5 Name:______________________________________ Section _________________ Date _________________ COLLIGATIVE PROPERTIES Data Water Solution A Solution B Solution C soln C cont’d Time Temp Time Temp Time Temp Time Temp Time Temp (min) sec °C (min) sec °C (min) sec °C (min) sec °C sec °C 30 30 30 30 930 960 1 60 1 60 1 60 1 60 90 90 90 90 990 1020 2 120 2 120 2 120 2 120 150 150 150 150 1050 1080 3 180 3 180 3 180 3 180 210 210 210 210 1110 1140 4 240 4 240 4 240 4 240 270 270 270 270 1170 5 300 5 300 5 300 5 300 1200 1230 330 330 330 330 6 360 6 360 6 360 6 360 1260 1290 390 390 390 390 7 420 7 420 7 420 7 420 1320 1350 450 450 450 450 8 480 8 480 8 480 8 480 1380 1410 510 510 510 510 9 540 9 540 9 540 9 540 1440 1470 570 570 570 570 10 600 10 600 10 600 10 600 1500 1530 630 630 630 630 11 660 11 660 11 660 11 660 1560 690 690 690 690 1590 1620 12 720 12 720 12 720 12 720 750 750 750 750 1650 1680 13 780 13 780 13 780 13 780 810 810 810 810 1710 14 840 14 840 14 840 14 840 1740 870 870 870 870 1770 15 900 15 900 15 900 15 900 1800 Table 1: Data Collection Table 6 Name:______________________________________ Section _________________ Date _________________ COLLIGATIVE PROPERTIES Data Temperature of ice/salt bath: ___________ Freezing point of water: ___________ Data Table Solution f.p.