Freezing Point Depression - Kf for Stearic Acid and Molar Mass of an Unknown Solid
Introduction
Procedure:
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In an earlier experiment you observed the change of vapor pressure of a liquid as a function of temperature. If a nonvolatile solid compound (the solute) is dissolved in a liquid, the vapor pressure of the liquid solvent is lowered. This decrease in the vapor pressure of the solvent results in other easily observable physical changes; the boiling point of the solution is higher than that of the pure solvent, and the freezing point is lower.

Many years ago chemists observed at low solute concentrations, the changes in the boiling point, the freezing point, and the vapor pressure of a solution are all proportional to the amount of solute that is dissolved in the solvent. These three properties are collectively known as colligative properties of solutions. The colligative properties of a solution depend only on the number of solute particles present in a given amount of solvent and not on the kind of particles dissolved.

When working with boiling point elevations or freezing point depressions of solutions, it is convenient to express the solute concentration in terms of its molality m defined by the relation:
 
For this unit of concentration, the boiling point elevation, Tb - Tºb or DTb, and the freezing point depression, Tºf - Tf  or  DTf,  in ºC at low concentrations are given by the equations: 
DTb = kbm     DTf = kfm    
where kb and kf are characteristic of the solvent used. For water, kb = 0.52 and kf = 1.86. For benzene, kb = 2.53 and kf = 5.10.

One of the main uses of the colligative properties of solutions is in connection with the determination of the molar masses of unknown substances. If we dissolve a known amount of solute in a given amount of solvent and measure DTb or DTf of the solution produced, and if we know the appropriate k for the solvent, we can find the molality and hence the molar mass, MM, of the solute. In the case of the freezing point depression, the reaction would be:
 
In this experiment you will be asked to estimate the molar mass of an unknown solute, using this equation. The solvent used will be stearic acid, which has a convenient melting point and a moderately large value for kf. The freezing points will be obtained by studying the rate at which liquid stearic acid and some of its solutions containing the unknown cool in air.

When a pure substance that melts at, say 70°C is heated to 80°C, where it will be completely liquid, and then allowed to cool in air, the temperature of the sample will vary with time, as in Figure 1. Initially the temperature will fall quite rapidly. When the freezing point is reached, solid will begin to form, and the temperature will tend to hold steady until the sample is all solid. The freezing point of the pure liquid is the constant temperature observed while the liquid is freezing to a solid.
Figure 1.
Figure 1.

Idealized cooling curves of a pure substance
and of a mixture of two substances.
The cooling behavior of a solution is somewhat different from that of a pure liquid, and is also shown in Figure 1. The temperature at which the solution begins to freeze is lower than for the pure solvent. In addition, there is a slow gradual fall in temperature as freezing proceeds. The best value for the freezing point of the solution is obtained by drawing two straight lines connecting the points on the temperature-time graph. The first line connects points where the solution is all liquid. The second line connects points where solid and liquid coexist. The point where the two lines intersect is the freezing point of the solution. With both the pure liquid and solutions, at the time when solid first appears, the temperature may fall below the freezing point and then come back up to it as solid forms. The effect is called supercooling, and is shown in Figure 1. When drawing the straight line in the solid-liquid region of the graph, ignore points where supercooling was observed. To establish the proper straight line in the solid-liquid region, it is necessary to record the temperature until the trend with time is smooth and clearly established.

A. PURPOSE
Find answers to the following questions:

1. What is the freezing point depression constant, Kf, for stearic acid used in this experiment?
2. What is the molar mass of an unknown solid based?
B. EXPERIMENTAL PROCEDURE

Make copies of the following two links.
DATA AND CALCULATION
ADVANCED STUDY ASSIGNMENT

1. Determine the Freezing Point of Stearic Acid
From the workbench at the front of the laboratory obtain a large test tube, wire stirring rod, 600-mL beaker, and thermistor. Use this equipment to assemble your set-up such that it will finally be like the one shown in Figure 2. The housing of the thermistor is made of glass and can easily be broken.  
Fill the 600-mL beaker about 75% full of deionized water, add a single boiling chip, and begin heating. Once the water comes to a rapid boil, reduce the heat sufficiently to maintain just a gentle boil. While the water is
Final stage in the melting the Stearic Acid Using the thermistor to measure the temperature as the liquid stearic acid freezes.
Figure 2.

Stearic Acid in the liquid state and just before removal from the heat source.
Figure 3.

Obtaining the temperature - time data to construct the cooling curve for stearic acid.
heating, weight your test tube on an analytical balance to a tenth of a milligram. Add about 10 grams of stearic acid to the test tube and weigh again to a tenth of a milligram. Once the solid stearic acid is molten, turn off the Bunsen burner. Remove the test tube containing the liquefied stearic acid from the beaker of boiling water and arrange the test tube, burette clamp, and thermistor as shown in Figure 3.

Use LabWorks II to record the temperature of the stearic acid as it cools in the air. Instructions on using LabWorks II with the thermistor to measure the temperature can be found in the accompanying pages. Stirring the liquid leads to irregular and inconsistent temperature time curves, so for best results do not agitate the liquid once its temperature drops to 80oC. The initial temperature of the liquefied stearic acid may vary anywhere from ~95 oC to 80oC. After the apparatus is in place as shown in Figure 3, start the computer recording and continue until the liquid has solidified(~ 700 seconds). Near the freezing point you will begin observing solid stearic acid in the liquid, and the solid phase will increase in amount as cooling proceeds until the stearic acid is a solid mass. After collecting your data and ending this portion, click the Save Data button to store the information to your ZIP disk or on the desktop. Again use a descriptive label:
Your Names-The experiment Part 1 Trial #WW.
(To go to a discussion of collecting the data, click here. )
2.Determining the Freezing Point of a Solution of pDCB and Stearic Acid
Weigh 1 gram of p-dichlorobenzene, Cl2C6H4, (Molar Mass = 147.002) on a piece of weighing paper on the analytical balance to the tenth of a milligram. Pour the p-dichlorobenzene, pDCB, into the test tube of solid stearic acid containing the entrapped thermistor* and stirring rod. Reweigh the paper. Be careful in transferring the pDCB, and with both weighings. Heat the test tube containing the entrapped thermistor* in the water bath until the stearic acid and pDCB are liquefied.

* Caution: The black plastic covering the wires of the thermistor is Polyvinyl Chloride (PVC). PVC has a low melting temperature and if PVC comes into contact with the hot objects such as the iron ring holding the wire gauze, the flame of the bunsen burner, the PVC will melt and damage the wires leads of the thermistor. IT IS YOUR RESPONSIBILITY TO SEE THAT THIS DOES NOT HAPPEN.

Make sure that all the stearic acid on the wall is melted; stir to mix the pDCB with the stearic acid thoroughly. Once the stearic acid and pDCB are molten and mixed, turn off the Bunsen burner. Remove the test tube containing the liquefied mixture and arrange the test tube, burette clamp, and thermistor as shown in Figure 3.

The initial temperature of the liquefied solution may vary anywhere from ~95 oC to 80oC. After the apparatus is in place as shown in Figure 3, start the computer recording and continue until the liquid mixture has solidified (~ 800 seconds). Stirring the liquid leads to irregular and inconsistent temperature time curves, so for best results do not agitate the liquid once the measurements are being made. Near the freezing point you will begin observing solid in the liquid, and the solid phase will increase in amount as cooling proceeds until the mixture is a solid mass. The dependence of temperature on time with the solution will be similar to that observed for the stearic acid, except that the first crystals will appear at lower temperatures, and the temperature of the solid-solution system will gradually fall as cooling proceeds. After collecting your data and ending this portion, click the Save Data button to store the information to your ZIP disk or on the desktop. Again use a descriptive label:
Your Names-The experiment Part 2 Trial #AA.

Repeat this portion of the experiment, weigh a second 1-gram sample of p-dichlorobenzene on a piece of weighing paper on the analytical balance to the tenth of a milligram. Add it to the test tube containing the pDCB and stearic acid. Reweigh the paper. Be careful in transferring the pDCB and with both weighings. Melt the mixture as before, heating it to above 80oC before removing the water bath. Repeat the entire procedure described above, start the computer recording and continue until the liquid mixture has solidified(~ 900 seconds). After collecting your data and ending this portion, click the Save Data button to store the information to your ZIP disk or on the desktop. Again use a descriptive label:
Your Names-The experiment Part 2 Trial #ZZ.
(To go to a discussion of collecting the data, click here. )

3. Cleaning the test tube and thermistor
Before Part D of this experiment, we must clean the test tube used in part A and B. Simply place the test tube into the 600-mL beaker containing boiling water. Once the solid has melted remove the very hot test tube out of the beaker using paper toweling. Immediately pour the molten material into the waste container found in the exhaust hoods. Place the test tube back into the boiling water. Once the stearic acid/pDCB is liquid, use a plastic disposable pipette to add the boiling water INTO the test tube until it is full. Again using paper as a hot-pad, remove the test tube from the boiling water and place the water-organic layer into the container provided by the instructor. DO NOT pour in to any of the sink drains! Repeat this latter process once more. Then using paper-toweling remove any remaining traces of organic from the test tube and make sure the test tube is dry.
4. Determining the Freezing Point of Stearic acid and Unknown Solute.
Add about 10 grams of stearic acid to the clean dry test tube and weigh again to a tenth of a milligram. Weigh 1 gram of unknown on a piece of weighing paper on the analytical balance to the tenth of a milligram. Transfer the 1 gram of unknown to the test tube containing the stearic acid. Re-weigh the paper. Be careful in transferring the unknown and with both weightings. Heat the test tube in the water bath until mixture is melted and the unknown substance is dissolved. Make sure that all the stearic acid on the wall is melted; stir the liquid solution to mix the unknown with the stearic acid thoroughly. Again the initial temperature of the liquefied solution may vary anywhere from ~95 oC to 80oC. After the apparatus is in place as shown in Figure 3, start the recording and continue until the liquid mixture has solidified (~ 800 seconds). Stirring the liquid leads to irregular and inconsistent temperature time curves, so for best results do not agitate the liquid once the measurements are being made. Near the freezing point you solid will appear in the liquid, and the solid phase will increase as cooling proceeds until the mixture is a solid mass. The temperature - time relationship will be similar to that observed for the stearic acid, except that the first crystals will appear at lower temperatures, and the temperature of the solid-solution system will gradually fall as cooling proceeds. After collecting your data and ending this portion, click the Save Data button to store the information to your ZIP disk or on the desktop. Again use a descriptive label:
Your Names-The experiment Part 4 Trial XX.

Repeat this portion of the experiment, weigh a second 1-gram sample of unknown on a piece of weighing paper on the analytical balance to the tenth of a milligram. Pour it into the test tube containing the unknown and stearic acid. Reweigh the paper. Be careful in transferring the unknown with both weighings. Melt the mixture as before, heating it to above 80oC before removing the water bath. Repeat the entire procedure described above, start the computer recording and continue until the liquid mixture has solidified(~ 900 seconds).. After collecting your data and ending this portion, click the Save Data button to store the information to your ZIP disk or on the desktop. Again use a descriptive label:
Your Names-The experiment Part 4 Trial #YY.
(To go to a discussion of collecting the data, click here. )
5. Final Cleaning of the Glassware. 
When you have completed the experiment, melt the stearic acid solution as directed in part 4. In addition, clean the 600-mL beaker. Fill the beaker with water, bring the water to boiling, with care, pour the water in the waste container found in the exhaust hood. Then using paper-toweling remove any remaining traces of organic material from the beaker and make sure it is dry.

Turn in both the clean test tube and the clean 600-mL beaker to the instructor for inspection and have the instructor sign your data sheet.

Make copies of the following two links before the laboratory period.
DATA AND CALCULATION
ADVANCED STUDY ASSIGNMENT

Instructions for the use of LabWorks II

- Computer-based measurements (must be completed during the laboratory period).

i. Opening LabWorks II .
To open the LabWorks II program:

either Click on the Microsoft start button in the lower left corner of the screen. when a popup screen appears, click PROGRAMS, and then drag the cursor over to LabWorks II 4. Another pop-up window will appear; then click on LabWorks.exe. The LabWorks II program is then activated.
or open LabWorks II 4 by clicking on the ICON found on the Desktop Window.

A view of LabWorks II Main Menu window should appear on your screen.
In the Main Menu screen on the Tool bar , open the Calibrate window by clicking on the button labeled Calibrate.

ii. Calibration Set-up
The Calibration window appears showing numerous probes which can be utilized in LabWorks II. In this experiment, we will use only thermistor as a probe:
We will complete three tasks to use the themistor for temperature measurements:

1st    Connect the probe to the LabWorks II interface
2nd  Edit the Sensor so that the data is collected as we choose
3rd  Calibrate the sensor using the Calibration Wizard. The sensor will be calibrated by recording a low temperature (ice water) and warm water (hot tap water) using a digital therometer.

iii. Design Set-up
The next step involves the design of the experiment. At this time you give the computer instructions to obtain temperature measurements at some regular time interval.

iv. Acquire Set-up
The next step involves collecting the experimental data using the probe and parameters previously defined.


v. Analysis Set-up View

The data you have collected will is now saved on your ZIP disk or to the desktop. To finish this experiment, you will now analyze this information.

We must accomplish a number of tasks:

Find and open the saved data file,
Open the Analyze window and make the following adjustments,

Set the factors influencing the form of the Graphed data,
      Define the General appearance of the Graph,
      Define the appearance of the X-axis,
      Define the appearance of the Y1-Axis,
      Define the appearance of the Y2-Axis,

Analyze our experimental numbers.
      Instructions on graphing in terms of the
assignments for the x- and y-axis.
      Use the LabWorks II curve fitting ability to
             analyze our data to obtain both the maximum
             and minimum temperatures.


iv. Print

Make certain the Spreadsheet and Graph view is on the computer screen, to print your graph. Click on the File button found on the menu toolbar. Click on SELECT PRINT; select USER GRAPH
Only print the User Graph rather than the Data Spreadsheet or User Spreadsheet. WHY?

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