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At-Home Chemistry Experiment

Colligative Properties: 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,

∆𝑇 = 𝐾𝑓 ∙ 𝑚 (1) Here,

∆𝑇 = 𝑇° − 𝑇 (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

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𝑚𝑜𝑙𝑎𝑙𝑖𝑡𝑦, 𝑚 = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑙𝑒𝑠 𝑠𝑜𝑙𝑢𝑡𝑒

𝑘𝑖𝑙𝑜𝑔𝑟𝑎𝑚𝑠 𝑠𝑜𝑙𝑣𝑒𝑛𝑡

(3)

𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠, 𝑀𝑀 = 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒

𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒

(4)

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

25 mL or 50 mL syringe

3 containers to store dissolved sucrose solutions (A, B and C). Each container should hold a minimum of 100 mL

Large container that can hold approximately 400-600 mL to be used for the ice/salt bath

12 Tablespoons of white granulated sugar (sucrose)

50 mL polypropylene tube

Water

Ice (crushed or cubed)

Salt (table salt, or kosher salt / coarse salt / de-icing salt), enough to achieve a 2-3 cm layer in chosen 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.

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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)

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.

Figure 2: a) Ice/Salt Bath, b) Temperature of Ice/Salt Bath

a) b)

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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)

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.

Figure 3: a) Polypropylene tube, b) measuring solution's freezing point, c) frozen sucrose solution

a) b) c)

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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).

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)

Figure 4: Sample data for the freezing point of water

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Name:______________________________________ Section _________________ Date _________________

COLLIGATIVE PROPERTIES Data

Water Solution A Solution B Solution C soln C cont’d

Time Temp °C

Time Temp °C

Time Temp °C

Time Temp °C

Time Temp °C (min) sec (min) sec (min) sec (min) sec sec

1

30

1

30

1

30

1

30 930

60

60

60

60 960

2

90

2

90

2

90

2

90 990

120

120

120

120 1020

3

150

3

150

3

150

3

150 1050

180

180

180

180 1080

4

210

4

210

4

210

4

210 1110

240

240

240

240 1140

5

270

5

270

5

270

5

270 1170

300

300

300

300 1200

6

330

6

330

6

330

6

330 1230

360 360 360 360 1260

7

390

7

390

7

390

7

390 1290

420 420 420 420 1320

8

450

8

450

8

450

8

450 1350

480

480

480

480 1380

9

510

9

510

9

510

9

510 1410

540 540 540 540 1440

10

570

10

570

10

570

10

570 1470

600 600 600 600 1500

11

630

11

630

11

630

11

630 1530

660

660

660

660 1560

12

690

12

690

12

690

12

690 1590

720

720

720

720 1620

13

750

13

750

13

750

13

750 1650

780

780

780

780 1680

14

810

14

810

14

810

14

810 1710

840

840

840

840 1740

15

870

15

870

15

870

15

870 1770

900 900 900 900 1800

Table 1: Data Collection Table

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Name:______________________________________ Section _________________ Date _________________

COLLIGATIVE PROPERTIES Data

Temperature of ice/salt bath: ___________ Freezing point of water: ___________ Data Table

Solution f.p. T m sucrose kg water n sucrose g sucrose MM sucrose % error

A

B

C

Average

Table 2: Data Analysis Table

Sample Calculations (show a set of sample calculations for one of the solutions):

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Troubleshooting Suggestions

Container for ice/salt bath could be a Pyrex measuring cup or a food thermos, or any container large enough to hold the ice/salt bath and polypropylene tube. It should be big enough so that when the tube is immersed into the ice/salt bath, the liquid level inside the tube is below the level of the ice/salt mixture.

If sugar solutions do not dissolve immediately, leave them out to dissolve over several hours (ideally prepare and let dissolve overnight). Do not heat them as you will lose some of the water by evaporation.

If the ice/salt bath level is lower than the liquid in the polypropylene tube, add more ice/salt to the bath.

Make sure you clean the polypropylene tube with warm water to dissolve and melt the frozen sucrose solution before moving on to the next sample.

The higher the concentration of the sucrose, the longer it takes to freeze. It may take up to 30 min to complete the measurements of the most concentrated solutions. Alternatively, you may place the higher concentration solutions in the fridge to cool down before performing the experiment to reduce the run time.

References Department of Chemistry. (2019, Fall). ‘Colligative Properties’, Chemistry of Solutions 202-NYB-05 Laboratory Experiments. Montreal, QC: Dawson College. Thompson, R. B. (2012). “Determine Molar Mass by Freezing Point Depression”, Illustrated guide to home chemistry experiments: all lab, no lecture. " O'Reilly Media, Inc.".

Rogers, C., (2004, Winter). CHEM 206 Tutorial Sample Problems, Montreal, QC: Concordia University

Acknowledgements Dawson College CLAW (Chemistry Lab Alternative Workforce) Elie Saadé, TAV College; for access to laboratory equipment and chemicals, as well as useful suggestions and insight into experimental procedures.

Last updated June 22, 2020

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Prepare your own Homemade Ice Cream (optional) Make your own ice cream at home using the same principle of freezing point depression examined in this experiment, “Ice Cream in a Bag” (Delish.com)

Reference: Abraham, Lena. “You Can Make Homemade Ice Cream In A Bag, And We've Lost All Chill.” Delish, Delish, 21 May 2019, www.delish.com/cooking/recipe-ideas/recipes/a54721/ice-cream-in-a-bag-recipe/.