Materials Included In Kit

Additional Materials Required

Safety Precautions

2,6-Di-tert-butyl-4-methylphenol, BHT, is moderately toxic by ingestion and inhalation and is a body tissue irritant. The stearic acid and cetyl alcohol are slightly toxic by ingestion and are body tissue irritants. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulation that may apply, before proceeding. Place the test tubes in a hot water bath until the mixtures are melted. Pour the melted substance out onto crumpled newspaper or paper towels. The paper towels and solid organic mixtures may be disposed of according to Flinn Suggested Disposal Method #26a. Rinse test tubes with acetone or ethanol before using detergent and water to clean them.

Lab Hints

• Use thermometers graduated to 0.1 °C. These thermometers are expensive but give the best data. The use of a thermometer with 1 °C divisions results in fewer significant digits. Digital thermometers also work well.
• Use copper or nichrome wire to make a stirring rod. Stirring is important to maintain a constant temperature throughout the solution.
• Stearic acid may be used in place of cetyl alcohol as the known substance to find the freezing point depression constant. BHT is convenient as the solvent because it is not very volatile, does not have a strong odor, and its melting point occurs at an easily measured temperature.
• Caution students to use the split rubber stopper only to support the thermometer and not to seal the test tube. Data are obtained most readily with a team of two students. Students should estimate temperature as closely as the thermometer in use allows.
• It is sometimes difficult to see the melting point using the capillary tubes. The solid in the tube should become transparent and liquid at the melting point. Some mixtures start out as almost transparent crystals. The use of the macroscale procedure allows a graphical technique for finding the melting point and yields more accurate results.

Teacher Tips

• The temperature data can be conveniently collected by a temperature probe interfaced with a computer, or with a CBL^® system. Using this type of technology allows more data points to be collected in a short time interval.
• Use a data collecting mode with the CBL or computer that automatically records both time and temperature. Record the temperature every 5 or 6 seconds as the substances are cooled. Record for about 8 minutes.
• If the data are collected manually, they may be graphed using a spreadsheet, or may be entered into a program, such as Graphical Analysis for Windows, and then printed out in graphical form.

Further Extensions

AP^® Standards

This lab fulfills the requirements for the College Board recommended AP experiment #4: Determination of Molar Mass by Freezing Point Depression. In addition, this lab provides the recommended familiarity with the observation and recording of phase changes.

Answers to Prelab Questions

1. The following data were obtained in an experiment designed to find the molar mass of a solute by freezing point depression.

   Solvent: para-dichlorobenzene
   Freezing point of pure solvent: 53.02 °C
   Mass of unknown substance: 2.04 g
   Freezing point depression constant: 7.1 °C/m
   Mass of para-dichlorobenzene: 24.80 g
   Freezing point of solution: 50.78 °C

   1. Determine the freezing point depression, ΔT_fp.

      ΔT_fp = (53.02 – 50.78)°C = 2.24 °C

   2. Using Equation 4, calculate the molar mass of the unknown substance.
      {13863_Answers_Equation_5}
2. What are colligative properties?

   Colligative properties are those that depend on the number of particles dissolved in solution and not on the type of dissolved particle.

Sample Data

Calculations

1. Freezing point depression constant for BHT, k_fp
   {13863_Data_Equation_6}
2. Molar mass of unknown, M
   {13863_Data_Equation_7}
3. Percent error in molar mass (Obtain actual molar mass value from your instructor.)

   Molar mass of stearic acid = 284.49 g/mol.

   {13863_Data_Equation_8}

Microscale Procedure Masses Calculations

k_fp, BHT, °C/m

k_fp = 7.2 °C/m

Molar mass, unknown

Molar mass = 300 g/mole

Answers to Questions

1. The following errors occurred when the above experiment was carried out. How would each affect the calculated molar mass of the solute (too high, too low, no effect)? Explain your answers.
   1. The thermometer used actually read 1.4 °C too high.

      The same thermometer was used for both the pure solvent and the solution. It is assumed that the thermometer read 1.4 °C too high for both pure solvent and for solution. The change in temperature was used to find the molecular mass, and should be accurate. The molar mass should be correct.

   2. Some of the solvent was spilled before the solute was added.

      The kilograms of solvent recorded was greater than the value actually used. Since kilograms of solvent appears in the denominator of the calculation used to find molar mass, the calculated value will be too small.

   3. Some of the solute was spilled after it was weighed and before it was added to the solvent.

      The grams solute recorded was larger than the amount actually used. Since grams solute appears in the numerator of the calculation for molar mass, the calculated molar mass will be too large.

   4. Some of the solution was spilled after the solute and solvent were melted but before the freezing point was determined.

      This will have no effect on the molar mass since the correct ratio of solute to solvent is known.

2. What was the least precise measurement in the experiment? How does this limit your significant digits?

   The temperature measurements are the least precise. Only two significant digits are allowed with these data.

3. Did your solutions show any evidence of supercooling?

   Some of the solutions did show supercooling.

4. Why is it advantageous to choose a solvent that has a large value for k_fp?

   The larger the value of the freezing point depression constant, the more precise the molecular mass can be with a small value for temperature change.

5. Explain why the pure solvent shows a level horizontal curve as solidification occurs, but the curve for the solution slopes downward slightly.

   A pure substance maintains a constant temperature as it freezes. When the solution freezes, the pure solvent freezes first. As it solidifies, the remaining solution is more concentrated so its freezing point is lower.

Introduction

A procedure for determining the molar mass of a substance is very useful to chemists. The molar mass is an important value that must be known in order to identify an unknown substance or to characterize a newly prepared compound.

Concepts

• Molality
• Colligative properties
• Freezing point depression

Background

There are a number of ways of determining the molar mass of a substance. One of the simplest involves finding the change in the freezing point of a solvent when an unknown substance is dissolved in it. The change in freezing point is directly proportional to the molality of the solution. This change in freezing point is one of several “colligative” properties of solutions—properties that depend only on the number of dissolved particles in solution, and not on the type of particle. Other colligative properties include changes in boiling point, vapor pressure and osmotic pressure. Measurements of these properties can be used to find the molar mass of solutes.

The molality of a solution, m, is defined as the moles of solute divided by the kilograms of solvent:

Since the moles of solute is the same as the grams of solute divided by the molar mass of the solute, then: The relation of molality to change in freezing point is: where ΔT_fp is the change in freezing point of the pure substance versus the solution, k_fp is the freezing point depression constant for the solvent, and m is the molality of the solution. The value of k_fp must be determined for each solvent.

Equations 2 and 3 are combined to solve for the molar mass of the solute. The solvent used in this experiment is a nonpolar solvent with the common name butylated hydroxytoluene. This compound is abbreviated BHT and is frequently used as an antioxidant in foods. The IUPAC name for the compound is 2,6-di-tert-butyl-4-methylphenol. Its structural formula is: The freezing point of BHT is approximately 70 °C. If the freezing points are determined for both the solvent and the solution using a thermometer with minor scale divisions marked every 0.1 °C, the freezing points can be estimated in the range: ±0.02 °C.

Figure 1 shows cooling curves obtained for both a pure solvent and for a solution. Notice that supercooling occurs in both the solvent and the solution. When supercooling occurs, the temperature falls below the freezing point until the first crystal forms. The temperature then rises up and either stays at the freezing point, in the case of the pure substance, or slowly falls as the solution freezes. The freezing point temperature T_fp of the solution is extrapolated from the graph.

Experiment Overview

The purpose of this experiment is to determine the molar mass of an unknown substance by measuring the freezing point depression of a solution of the unknown substance and BHT. The freezing point of BHT is first determined. Even though the freezing point of butylated hydroxytoluene is known, it is necessary to determine it with the thermometer that is used in the experiment. Thermometers can give temperature readings that are slightly different from true values. Even if the thermometer reading is slightly off, the change in temperature should be accurate. It is important that the same thermometer is used to determine both the freezing point temperature of the solvent and that of the solution.

A known amount of cetyl alcohol is then added to a measured quantity of BHT. The freezing point depression of this solution is found and the freezing point depression constant (k_fp) is calculated. The unknown is added to BHT, the freezing point depression of this solution is measured, and the molar mass of the unknown is then determined.

Materials

Prelab Questions

1. The following data were obtained in an experiment designed to find the molar mass of a solute by freezing point depression.

   Solvent: para-dichlorobenzene
   Freezing point of pure solvent: 53.02 °C
   Mass of unknown substance: 2.04 g
   Freezing point depression constant: 7.1 °C/m
   Mass of para-dichlorobenzene: 24.80 g
   Freezing point of solution: 50.78 °C

   1. Determine the freezing point depression, ΔT_fp.
   2. Using Equation 4, calculate the molar mass of the unknown substance.
2. What are colligative properties?

Safety Precautions

2,6-Di-tert-butyl-4-methylphenol, BHT, is moderately toxic by ingestion and inhalation and is a body tissue irritant. Cetyl alcohol and the unknown substance are slightly toxic by ingestion and are body tissue irritants. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

1. Assemble the apparatus as diagrammed in Figure 2. Do not add water to the beaker. Clamp the thermometer using a split rubber stopper. Do not seal the test tube with the stopper—it is just to support the thermometer. Make a stirrer out of wire bent with a circle at the bottom. The test tube is clamped in the beaker so that the solid it contains will be below the level of the water in the beaker. The beaker sits on a hot plate.
   {13863_Procedure_Figure_2_Diagram of apparatus for freezing point determination}
2. Disassemble the apparatus by sliding both the thermometer and stirring wire assembly and the test tube clamp off the ring stand. Weigh the test tube on an analytical balance.
3. Accurately measure about 8 g of BHT into the test tube. Record the combined mass of the test tube and the BHT in the data table.
4. Clamp the test tube in the beaker and insert the thermometer and stirring wire assembly into the test tube and clamp the assembly. Do not force the thermometer into the solid—allow it to sit on top of the solid. Add water to the beaker so that the solid in the test tube is well below the level of the water.
5. Turn on the hot plate and heat the water bath to about 90 °C.
6. Allow the BHT in the test tube to melt.
7. When the temperature of the BHT is 80 °C or above, remove the thermometer and test tube from the water bath. First, raise the thermometer clamp so that the thermometer bulb is slightly higher than the height of the water bath beaker. Next, raise the test tube clamp so that the thermometer is still positioned correctly in the test tube and the test tube clears the water bath beaker (see Figure 3).
   {13863_Procedure_Figure_3}
8. Record the BHT temperature in the Cooling Data Table every 20 seconds as the melted BHT cools. It is important to continuously stir the BHT to maintain even cooling. Stirring also helps prevent supercooling. Stir until BHT solidifies.
9. Continue recording temperature values in the Cooling Data Table until at least five values are constant. Make a note of the temperature at which crystals begin to form.
10. If instructed, repeat this measurement.
11. Using an analytical balance, accurately measure about 1 g of cetyl alcohol onto a piece of waxed weighing paper and record its exact mass in the data table.
12. Place the cetyl alcohol into the test tube containing BHT.
13. Clamp the test tube in the water bath and insert the thermometer and stirring wire assembly into the test tube (see Figure 2).
14. Heat the mixture in the hot water bath until the substances are all melted. Stir well to ensure the solution is homogeneous.
15. When the solution temperature reaches 80 °C or higher, remove the test tube from the hot water bath. Stir the solution and record the temperature every 20 seconds as the solution cools. Record at least six temperature values after crystals first begin to form.
16. If instructed, repeat this measurement.
17. Repeat steps 1–3 and 11–15 using fresh BHT, a clean test tube, thermometer, stirrer, and about 1 g of the unknown compound in place of 1 g of cetyl alcohol. Repeat this measurement, if instructed.
18. Your instructor will provide disposal and clean-up instructions.

Alternative Microscale Procedure

1. Pulverize a small amount (about 0.5 g) of BHT. Use a mortar and pestle, or use a watch glass and the bottom of a test tube.
2. Pack the BHT in a capillary tube to a depth of about 1 cm. To get the BHT into the capillary tube, push the open end of the capillary tube down into a small pile of BHT powder. Turn the tube so the open end is up, and bounce the bottom of the tube on the desk top. The BHT may also be packed by holding a long piece of 6 mm diameter glass tubing (1 m in length) upright on the floor, and dropping the capillary tube down the glass tubing so it bounces up and down a few times.
3. Obtain a small rubber band, and use it to fasten the capillary tube to a thermometer. The BHT should be level with the bulb of the thermometer.
4. Use a universal clamp and split rubber stopper to fasten the thermometer to a ring stand.
5. Immerse the bottom of the capillary and thermometer in a beaker of water (or a Thiele melting point tube filled with water) and heat (see Figure 4).
   {13863_Procedure_Figure_4}
6. If using a beaker, stir the water to maintain an even distribution of temperature. Heating can be rapid in the beginning, but as the temperature approaches the melting point, ≈ 67 °C, heat very slowly in order to get an accurate value.
7. Record the temperature at which the BHT melts (the white powder will become clear) in the data table. If repeating the melting procedure, use a new sample and capillary tube.
8. Use an analytical balance to accurately measure about 0.5 g BHT into a small beaker. Record the mass in the data table.
9. Accurately measure about 0.1 g cetyl alcohol into the beaker. Record the mass in the data table.
10. Heat the beaker gently over a small flame or on a hot plate.
11. When the substance melts, mix it well with a stirring rod until it is homogeneous.
12. Allow the mixture to cool and solidify.
13. After it solidifies, pulverize a small amount of the mixture. Repeat steps 2–7 to determine the solution melting point.
14. Repeat steps 8–13 using 0.5 g of BHT and 0.1 g of the unknown substance. 

Student Worksheet PDF

13863_Student1.pdf