Using Freezing-Point Depression

Science in Motion

Using Freezing-Point Depression to Find Molecular Mass

PA Standards:

3.4.12.A Apply concepts about the structure and properties of matter – Quantify the properties of matter by applying mathematical formulas.

3.7.12.A Apply advanced tools, materials and techniques to answer complex questions – evaluate and use technological resources to solve complex multistep problems.

3.7.12.B Evaluate appropriate instruments and apparatus to accurately measure materials and processes

– apply and evaluate the use of appropriate instruments to accurately measure scientific and technologic phenomena within the error limits of the equipment.

Background:

When a solute is dissolved in a solvent, the freezing temperature is lowered in proportion to the number of moles of solute added. This property, known as freezing-point depression, is a colligative property ; that is, it depends on the ratio of solute and solvent particles, not on the nature of the substance itself. The equation that shows this relationship is:

 T = K f • m where  T is the freezing point depression, solution (in mol solute/kg solvent).

K f

is the freezing point depression constant for a particular solvent (3.9°C-kg/mol for lauric acid in this experiment 1 ), and m is the molality of the

In this experiment, you will first find the freezing temperature of the pure solvent, lauric acid,

CH

3

(CH

2

)

10

COOH. You will then add a known mass of benzoic acid solute, C

6

H

5

COOH, to a known mass of lauric acid, and determine the lowering of the freezing temperature of the solution. In an earlier experiment, you observed the effect on the cooling behavior at the freezing point of adding a solute to a pure substance. By measuring the freezing point depression,  T, and the mass of benzoic acid, you can use the formula above to find the molecular mass of the benzoic acid solute, in g/mol.

Figure 1

1 “The Computer-Based Laboratory,” Journal of Chemical Education: Software , 1988, Vol. 1A, No. 2, p. 73.

Lab # 761, Freezing Point Depression to Find Molecular Mass , (5/24/07) by Advancing Science Program, Gettysburg College, Gettysburg, PA

17325. www.advancingscience.org

A Science in Motion Program

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Guiding Question(s):

What is the general shape of the cooling curve of a liquid as it freezes?

How does the addition of a solute (impurity) affect the cooling curve of a pure substance?

Vocabulary:

Solvent, solute, molality, colligative property, freezing point, molecular mass

Materials:

Macintosh or IBM-compatible computer

Lab Pro interface

Logger Pro software

Vernier Temperature Probe

400-mL beaker ring stand utility clamp

18

X

150-mm test tube lauric acid benzoic acid thermometer

Hot water bath

Safety:



Always wear safety glasses or goggles.



Melted lauric acid can cause severe burns. Please use extreme care when handling



Use caution around hot plates.

Procedure:

1. Obtain and wear goggles.

2. Prepare the computer for data collection by opening “Exp 15” from the Chemistry with

Computers experiment files of Logger Pro . The vertical axis of the graph has temperature scaled from 0°C to 100°C. The horizontal axis has time scaled from 0 to 10 minutes.

Part I Freezing Temperature of Pure Lauric Acid

3. Add about 300 mL of tap water with a temperature between 20 and 25°C to a 400-mL beaker.

Place the beaker on the base of the ring stand.

4. Use a utility clamp to obtain a test tube containing hot melted lauric acid from your teacher.

Fasten the utility clamp at the top of the test tube. CAUTION: Be careful not to spill the hot lauric acid on yourself and do not touch the bottom of the test tube.

5. Insert the temperature probe into the hot lauric acid. About 45 seconds are required for the probe to warm up to the temperature of its surroundings and give correct temperature readings. During this time, fasten the utility clamp to the ring stand so the test tube is above the water bath. Then click Collect to begin data collection.

6. If the lauric acid is not above 50°C, obtain another lauric acid sample and begin again. Lower the test tube into the water bath. Make sure the water level outside the test tube is higher than the lauric acid level inside the test tube.

7. Holding the top of the temperature probe (and not its wire) use a very slight up and down motion of the temperature probe, to continuously stir the lauric acid during the cooling.

8. Continue with the experiment until data collection has stopped after 10 minutes. Unplug the temperature probe and use the hot water bath provided by your teacher to melt the probe out of the solid lauric acid. Do not attempt to pull the probe out of the solid—this might damage




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Science in Motion it. Carefully wipe any excess liquid lauric acid from the probe with a paper towel or tissue.

Return the test tube containing lauric acid to the place directed by your teacher.

9. To determine the freezing temperature of pure lauric acid, you need to determine the mean

(or average) temperature in the portion of graph with nearly constant temperature. Move the mouse pointer to the beginning of the graph’s flat part. Press the mouse button and hold it down as you drag across the flat part of the curve, selecting only the points in the plateau.

Click on the Statistics button, . The mean temperature value for the selected data is listed in the statistics box on the graph. Record this value as the freezing temperature of lauric acid.

Click on the upper-right corner of the statistics box to remove it from the graph.

Part II Freezing Temperature of a Solution of Benzoic Acid and Lauric Acid

10. Prepare the computer for data collection. From the Data menu, choose Store Latest Run. This stores the data so it can be used later. To hide the curve of your first data run, click the

Temperature vertical-axis label of the graph, and uncheck the Run 1 box. Click OK .

11. Obtain a test tube containing a melted solution with ~1 g of benzoic acid dissolved in ~8 g of lauric acid. Record the precise masses of benzoic acid and lauric acid as indicated on the label of the test tube. Repeat Steps 3-8 to determine the freezing point of this mixture.

12. When you have completed Step 8, click on the Examine button, . To determine the freezing point of the benzoic acid-lauric acid solution, you need to determine the temperature at which the mixture initially started to freeze. Unlike pure lauric acid, cooling a mixture of benzoic acid and lauric acid results in a gradual linear decrease in temperature during the time period when freezing takes place. As you move the mouse cursor across the graph, the temperature (y) and time (x) data points are displayed in the examine box on the graph. Locate the initial freezing temperature of the solution, as shown in

Figure 2. Record the freezing point in the Data and

Calculations table. Time

Freezing Point

Figure 2

13. To print a graph of temperature vs. time showing both data runs:

• Click the Temperature vertical-axis label of the graph. To display both temperature runs, check the Run 1 and Latest boxes. Click OK .

• Label both curves by choosing Text Annotation from the Insert menu, and typing “Lauric acid” (or “Benzoic acid-lauric acid mixture”) in the edit box. Then drag each box to a position on or near its respective curve.

• Print a copy of the Graph window by choosing File → Print Graph. Enter your name(s) and any comments in the Print Options box. Choose the number of copies of the graph you want and select the appropriate printer. Click OK

.




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Science in Motion

Name:

Data Table and Results:

Mass of lauric acid

Date:__________________

________ g

Mass of benzoic acid

________ g

Freezing point of pure lauric acid

________ °C

Freezing point of the benzoic acid- lauric acid mixture

1) Freezing temperature depression,

2) Molality, m

3) Moles of benzoic acid

4) Molecular weight of benzoic acid

(experimental)

5) Molecular weight of benzoic acid

(accepted)

6) Percent error

 t

________ °C

________ °C

________ mol/kg

________ mol

________ g/mol

________ g/mol

________ %




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Science in Motion

Calculations (show all work below):

1. Determine the difference in freezing temperatures, the preceeding table.

 t, between the pure lauric acid (t

1

) and the mixture of lauric acid and benzoic acid (t

2

). Use the formula,  t = t

1

- t

2

. Enter results in

2. Calculate molality (m), in mol/kg, using the formula,  t = K f •

m (K f

= 3.9°C-kg/mol for lauric acid). Enter results in the preceeding table.

3. Calculate moles of benzoic acid solute, using the answer in Step 2 (in mol/kg) and the mass

(in kg) of lauric acid solvent. Enter results in the preceeding table.

4. Calculate the experimental molecular weight of benzoic acid, in g/mol. Use the original mass of benzoic acid from the Data and Results table, and the moles of benzoic acid you found in the previous step. Enter results in the preceeding table.

5. Determine the accepted molecular weight for benzoic acid from its formula, C

6

H

5

COOH.

Enter results in the preceeding table.

6. Calculate the percent error. Enter results in the preceeding table.

Extension:

Here is another method that can be used to determine the freezing temperature from your data in

Part II. With a graph of the Part II data displayed, use this procedure:

• Move the mouse pointer to the initial part of the cooling curve, where the temperature has an initial rapid decrease (before freezing occurred). Press the mouse button and hold it down as you drag across the linear region of this steep temperature decrease.

• Click on the Linear Regression button, .

• Now press the mouse button and drag over the next linear region of the curve (the gently sloping section of the curve where freezing took place). Press the mouse button and hold it down as you drag only this linear region of the curve.

• Click again. The graph should now have two regression lines displayed.

• Choose Interpolate from the Analyze menu. Move the mouse pointer left to the point where the two regression lines intersect. When the small circles on each cursor line overlap each other at the intersection, the temperatures shown in either examine box should be equal to the freezing temperature for the benzoic acid-lauric acid mixture.

Using Freezing Point Depression to Find Molecular Mass was adapted from (Lab #15) Chemistry with Computers , by Dan Holmquist and

Donald Volz. The lab was adapted by Jack Sipe for the Advancing Science Progrm at Gettysburg College, Gettysburg, PA 17325 http://www.advancingscience.org




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Science in Motion

Teacher Notes

Using Freezing Point Depression to Find Molecular Mass

Objectives:

 Use temperature probe, lab pro and computer to measure cooling curve of both a pure solvent (lauric acid) and a solution (lauric/benzoic acid).

 Determine the molecular mass of benzoic acid and compare to accepted value by calculating percent error.

Lab Time: 40 – 50 minutes, easily split into two periods

Grade Level: 10 – 12, chemistry

1. Lauric acid, CH

3

(CH

2

)

10

COOH, has an accepted melting point of 44.0°C. It is also called dodecanoic acid. It has an accepted molecular weight value of 122.0.

2. The



T value limits the accuracy of the final answer for molecular weight to two significant figures. As a result, the final molecular weight in the sample data was expressed as 120 g/mol, not 119 g/mol.

3. Test tube sizes 18

X

150 mm, 20

X

150 mm, and 25

X

150 mm work well.

4. In order to have plenty of time to run two trials in one 40-50 minute period, we recommend that you have pre-measured lauric acid and lauric acid-benzoic acid samples in test tubes. In

Part I, a sample of lauric acid that is approximately 8 grams (filled to a depth of about 10 cm) works well. In Part II, measure 8 g of lauric acid and 1 g of benzoic acid into each test tube, to the precision of your electronic balance (at least 0.01 g). Place a label on each test tube showing the precise masses of the lauric acid and the benzoic acid. Be sure the label will be above the water level of the water bath. Clear packing tape over the label helps too. Students will record and use these masses in their calculations.

5. A hot-water bath on a hot plate adjusted to a low setting (one that maintains the water bath at a temperature of 80-90°C) can be used to melt the lauric acid samples and maintain them in the liquid state. When initially preparing the lauric acid-benzoic acid mixture, the benzoic acid will not melt in the water bath. It is necessary to “flash melt” the benzoic acid over a

Bunsen burner flame. Once the benzoic acid is melt and mixed to form a homogenous solution, the solution is easily melted in the water bath. Place the beaker with samples of lauric acid on one hot plate at one location, and the beaker with samples of the lauric acidbenzoic acid mixtures on a second hot plate at another location.

6. If you follow the procedure in Steps 4 and 5 above, you can easily reuse samples of lauric acid and the lauric acid-benzoic acid mixtures. There should be very little contamination of the samples if you remind students to wipe any excess liquid from the probe at the end of a trial. This method also makes clean-up a great deal easier for you at the end of the lab.

Stopper the test tubes and store them for future use.




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7. Step 6 mentions at least one temperature reading of 50°C or greater. This ensures a portion of the graph showing the drop down to the 44°C freezing temperature of lauric acid.

8. Have a hot-water bath available to free probes that have become frozen in lauric acid (or the mixture). Students will have to disconnect the temperature probe from the Lab Pro. The test tubes can be immersed in the bath to melt the solution and free the probes. Be sure the wires do not get against the hot plates.

9. If you have class periods longer than 50 minutes, you may choose to have students weigh out their own samples of lauric acid and benzoic acid. We still recommend having hot plates with hot water baths available (as well as Bunsen burners and stirring rods) to save students the time to heat up a water bath.

10. HAZARD ALERT:

Benzoic acid: Moderately toxic by ingestion; irritates eyes, skin and respiratory tract; combustible. Hazard Code: C—Somewhat hazardous.

The hazard information reference is: Flinn Scientific, Inc., Chemical & Biological Catalog

Reference Manual, 2000, P.O. Box 219, Batavia, IL 60510. See Appendix D of this book,

Chemistry with Computers , for more information.

11. The stored calibrations for the Stainless Steel Temperature Probe work well for this experiment.

Sample results:

Mass of lauric acid

Mass of benzoic acid

Freezing temperature of pure lauric acid

Freezing point of the benzoic acid –lauric acid mixture

1) Freezing temperature depression,

2) Molality, m

 t

8.01 g

1.00 g

44.0°C

39.9°C

 t = 44.0°C – 39.9°C =

4.1°C m =  t / K f

= Ошибка!

=

1.05 mol/kg

3) Moles of benzoic acid

4) Molecular weight of benzoic acid (experimental)

(1.05 mol/kg)(0.00801 kg) =

0.00841 mol

Ошибка!

= 119 =

120 g/mol

5) Molecular weight of benzoic acid (accepted)

C: 7

X

12.0 = 84.0

H: 6

X

1.0 = 6.0

O: 2

X

16.0 = 32.0

122.0 122 g/mol




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6) Percent error

Science in Motion

Ошибка!

X

100 = 2 %

Pure lauric acid Lauric acid - benzoic acid mixture

9

10

11

12

13

14

15

16

17

18

19

20

21

1

2

3

4

5

6

7

8

Time minutes

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

9.0

9.5

10.0

Temp

 C

76.8

66.6

51.5

46.2

44.7

44.1

44.1

44.1

44.1

44.1

43.9

43.9

44.1

43.9

43.9

43.0

38.6

34.8

32.4

30.5

29.0

9

10

11

12

13

14

15

16

17

18

19

20

21

1

2

3

4

5

6

7

8

Time minutes

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

9.0

9.5

10.0

Temp

 C

39.2

38.8

38.8

38.4

38.2

37.7

37.1

36.7

88.0

55.3

42.0

39.9

39.7

39.7

39.4

39.4

36.5

36.5

35.8

35.0

34.1




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Using Freezing Point Depression to Find Molecular Mass was adapted from (Lab #15) Chemistry with Computers , by Dan Holmquist and Donald Volz. The lab was adapted by Jack Sipe for the Advancing Science Progrm at Gettysburg College, Gettysburg, PA 17325 http://www.advancingscience.org




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Advancing Science /SIM

Pre/Post Lab Quiz – Freezing Point Depression to Find Molecular Mass

1. Which of the following is NOT a reason the computer, Lab Pro and temperature probe system is used in place of a glass thermometer?

a) The temperature is measured digitally.

b) It is quicker and easier to set up.

c) A real time temperature graph is produced.

d) Stainless steel probes are safer to use.

2. Identify which of the following is a practical application of the freezing point depression concept. a) The application of road salt to snowy roads b) Adding dry gas to the gas tank c) Making ice from hot water rather than cold d) all of the above

3. The principle of Freezing Point Depression (Raoult’s Law) depends on the:

a) starting temperature of the solvent

b) nature of the solute

c) nature of the solvent

d) number of particles of solute

4. Please choose which of the following cooling curves best follows that of a pure substance as it freezes.

a) b) c) d)

time time time time

5. The equation,  T = K f • m show the relationship describing Freezing Point Depression.

Which of the three above quantities will be “measured” in the lab?

a)  T, change in temperature

b) K f, freezing point constant

c) m, the molality

d) all of the above

Lab # 761, Freezing Point Depression to Find Molecular Mass , (5/24/07) by Advancing Science Program, Gettysburg College,

Gettysburg, PA 17325. www.advancingscience.org


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Using Freezing-Point Depression

Using Freezing-Point Depression to Find Molecular Mass

Name:

Data Table and Results:

Date:__________________

Calculations (show all work below):

Extension:

Teacher Notes