PROCEDURES

Your instructor will set up the temperature probe interface to the computer and prepare the software for data acquisition. Be sure to familiarize yourself with the layout of the computer screen, pg. 10-14. You will need to be fully prepared to start recording data as soon as your sample is removed from the hot water bath; so knowing what to do with the computer is important. If you are not ready, the lauric acid will cool too much before you can start recording data.

Remember to make and record observations during the course of the experiment. Examples of good observations are appearance of reactants and products, colors, and phase changes.

Part A: Calibration of the Temperature Probe

1. Set up a 400-mL beaker on a ring stand equipped with a wire screen. Add about 350 mL of distilled water to the beaker and heat with a Bunsen burner until boiling.

2. Choose "CALLIBRATE" from the "EXPERIMENT" pull-down menu.

3. Select "PERFORM NOW".

4. Place the probe in the boiling water and use it to stir constantly for about 30 seconds. Grasp the stick portion of the probe; DO NOT hold the wire to stir with. Keep the wire away from the flame and hot ring stand, because damage to the insulation will ruin the probe.

5. When the value of “Input 1” in the column labeled “Reading 1” stabilizes, enter the value 100 and click the “KEEP” button.

6. Adjust the Bunsen burner to a lower flame to keep the water warm and remove the probe from the hot water.

7. Add 50 mL of distilled water to 250 mL of crushed ice and stir with the probe until the value of “Input 1” in the column labeled “Reading 2” stabilizes. Enter the value 0 (zero) and click “KEEP’ and then click “OK” to exit the calibration program.

8. The probe is now calibrated, but the calibration will be lost if you close or quit the program at any time. Consult your instructor if you have questions.

Part B: Freezing Temperature of Pure Lauric Acid

1. Obtain a large test tube from the side bench and use the large end of a clean scoopula to place about 8 grams of lauric acid into the test tube. Be careful and avoid spilling the powdered acid. Record the exact mass of acid to three decimal places (line 2, page 10-11).

2. Equip the test tube with two test tube holders. Overlap them so that they provide support for the test tube in the hot water bath.

3. Heat the acid in the water bath until it is fully liquefied. You may insert the temperature probe to gently stir the acid and hasten the dissolving. Once inserted, DO NOT remove the temperature probe until completion of Part B of the procedure, because removing the probe may cause loss of lauric acid. BE EXTREMELY CAREFUL HERE TO AVOID HAVING THE PROBE WIRE HIT EITHER THE FLAME OR HOT RING STAND. The insulation will melt and ruin the probe.

4. Prepare a second 400-mL beaker with 350 mL of distilled water. Cold water directly from the distilled water tap will be adequate.

5. The temperature of the liquefied acid can be read on the computer display even before the "COLLECT" button has been clicked. Look just below the graph display. When the temperature reaches about 80 ¾C, one partner should remove the test tube from the water bath and the other partner should immediately click "Collect". Caution: Be careful not to spill the hot lauric acid on yourself and do not touch the bottom of the test tube. The person removing the test tube should continuously stir the acid with a gentle motion of the temperature probe. Be sure to hold the probe by its top and not the wire. Stirring by flexing the wire will cause the wire to break.

6. After the computer trace reaches 30 seconds, lower the test tube into the cold water and continue to stir gently. Ensure that the water level in the beaker is higher than the level of the acid in the test tube. Make observations during the process.

7. After the data trace has been level for at least 300 seconds, you may stop stirring. The computer will continue to record data for a total of 10 minutes. Wait the full time. The program will automatically stop when the time has expired.

8. Return your test tube and probe to the hot water bath while you print your graphs.

9. Before printing, you should re-scale the temperature axis. This will allow a more accurate reading of the freezing point. First click on the data table to make it the active window. Note the highest and lowest temperatures on the table. Now make the graph the active window and select "Graph Options" from the "VIEW" pull-down menu, and finally click on the “Axis Options” tab. Add 1 to the highest temperature and enter the result as the maximum for the y-axis. Subtract 1 from the minimum temperature and enter the result as the minimum for the y-axis. Click “Apply” to see if your adjustments are satisfactory. If the flat portion of the curve is too close to the bottom or top of the graph, make a further 1-degree change in the minimum or maximum of the y-axis. Finally click "OK".

10. You are almost ready to print your graph. Select "Print" from the "FILE pull-down menu and input the names of the two partners on the first line and the title of your experiment, such as "Freezing Curve for Pure Acid," on the second line. Click “OK” and then enter the number of copies (one for each partner) before you click “Print” on the next pop up window. Your graph should look something like Figure 1.

Part C: Freezing Temperature of a Mixture of a Solute and Lauric Acid

1. Your instructor will assign an unknown solute. Remove the test tube and probe from the hot water bath. Carefully remove the temperature probe. Tap the side of the test tube gently with the probe to remove drops of lauric acid.

2. Lay the probe on a clean paper towel and re-weigh your test tube plus acid. (You may want to stand the test tube vertically in a small beaker for the weighing.)

3. Add about 1 gram of your assigned unknown to the acid in the test tube. Record the mass of the unknown to 3 decimal places (line 3, page 10-11). You now have a mixture that is about 12% solute by mass. The mixture should freeze at a lower temperature than the pure acid.

4. Return your test tube and mixture to the hot water bath, insert the temperature probe, and stir gently.

5. Repeat the procedure from Part B, remembering to stir constantly for at least 300 seconds. Remember to add your names and title (Freezing Point of Solution) to your graph when you print. Your graph should look something like Figure 2.

Part D: Waste Disposal and Clean-up

Return the test tube and probe to the hot water bath. When the mixture is completely liquefied, remove the probe. Carry the hot test tube to the disposal container and pour the liquid into the container. Use the squeeze bottle of acetone to rinse the test tube twice with 2 or 3 mL of acetone. The acetone must be poured into the container labeled "Acetone Waste". DO NOT combine the two different waste products. Place the rinsed test tube in the tray provided. Carefully rinse the temperature probe with acetone from a squeeze bottle. Two or 3 milliliters is sufficient. The waste should be poured into the "Acetone Waste" container. Dry the probe with a Kimwipe and lay the probe on a clean paper towel. Clean up your area by organizing and storing the equipment and washing glassware. Wash your hands.

CAUTION: acetone is flammable; extinguish all flames

Part E: Data Analysis

We encourage the use of pencils to prepare graphs. The freezing temperature of pure lauric acid is obtained from the graph of data from Part B. Use a ruler to draw the best straight line through the nearly horizontal points and then draw a second line angled vertically through the initial points. These are the points representing the rapid cooling just after the test tube is placed in cold water. The intersection of the two straight lines provides the best estimate of the freezing temperature. Read this temperature from your graph to the nearest 0.1 ¾C, and enter it on line 1 of the data page.

Follow the same procedure for Part C, except that the mid portion of the graph will probably be sloped, i.e. not horizontal. Draw the best straight line through those points, and a second, more vertical line through the initial points. Again, the intersection of the two lines gives the best estimate of the freezing temperature of the mixture. Record this temperature, to the nearest 0.1 ¾C on line 4 of the Data Page.

Complete the data and calculations to determine the experimental molar mass of the unknown solute, line 8 of the Data Page. Show the results to your instructor. Your instructor will initial the results and give you the molecular formula for your unknown. You can then calculate the theoretical molar mass, line 9, and the percent error, line 10. Be sure to complete the set-ups (formula or equation, data, completed calculation), and write your "Statement of Results and Conclusions" (Page 10-12)

POSSIBLE UNKNOWN SOLUTES FOR FREEZING POINT DEPRESSION
Name	Melting Point	Molar Mass g/mol
acetic acid	16.6	60.05
ethylene glycol	-17.4	62.07
tert-butyl alcohol	25.5	74.12
lactic acid	18.0	90.08
glycerol	17.9	92.09
cyclohexanol	24	100.16
2-hydroxypentanoic acid	34	118.13
methyl phenyl ketone 1 (acetophenone)	19.7	120.14
1,7-heptanediol (heptamethylene glycol)	12	132.26
octanoic acid (caprylic acid)	16	144.21
phenyl propyl ketone	11	148.2
1-decanol	6	158.28
diphenyl ether	28	170.20
n-decanoic acid (capric acid)	31.5	172.26
linoleic aci (9,12-octadecadienoic acid) (C18H32O2)	-11	280.44