# Molar Mass by Freezing Point Depression AP Chemistry Laboratory #4

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1 Catalog No. AP6356 Publication No. 6356A Molar Mass by Freezing Point Depression AP Chemistry Laboratory #4 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: moles (solute) m = Equation 1 kg (solvent) Since the moles of solute is the same as the grams of solute divided by the molar mass of the solute, then: g (solute) m = Equation 2 kg (solvent) molar mass (solute) The relation of molarity to change in freezing point is: T fp = k fp m Equation 3 where T 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. k fp g (solute) Molar mass (solute) = Equation 4 kg (solvent) T fp CHEM-FAX...makes science teaching easier. IN6356A

2 Molar Mass by Freezing Point Depression Page 2 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: OH (CH 3 ) 3 C C(CH 3 ) 3 CH 3 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 f of the solution is extrapolated from the graph. Temperature vs. Cooling Time for Solvent and Solution Ο Ο Ο Ο Pure Solvent Ο Ο Ο Ο Ο Ο Ο Ο Supercooling (a) = Freezing point of pure solvent (b) = Freezing point of solution ΔT fp Solution Time of Cooling Figure 1. Freezing Point Graph for Pure Solvent and for Solution 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.

3 Molar Mass by Freezing Point Depression Page 3 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. Pre-Lab 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: C Freezing point depression constant: 7.1 C/m Mass of para-dichlorobenzene: g Mass of unknown substance: 2.04 g Freezing point of solution: C a. Determine the freezing point depression, ΔT fp. b. Using Equation 4, calculate the molar mass of the unknown substance. 2. What are colligative properties? Materials 2,6-Di-tert-butyl-4-methylphenol, BHT, 16 g Cetyl alcohol, CH 3 (CH 2 ) 14 CH 2 OH, 1 g Unknown substance, 1 g Test tube, mm Beaker, 400-mL Universal clamps, 2 Split rubber stopper with one hole Timer, seconds Balance (0.001 g precision) Ring stand Wire stirrer Weighing paper or dish Thermometer (preferably graduated to 0.1 C) Hot plate or Bunsen burner, ring clamp and wire gauze with ceramic center 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 chemicalresistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

4 Molar Mass by Freezing Point Depression Page 4 Procedure Thermometer Split stopper Stirrer Water bath Hot plate Figure 2. Diagram of Apparatus for Freezing Point Determination 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. 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.)

5 Molar Mass by Freezing Point Depression Page 5 Thermometer Split stopper Stirrer Water bath Hot plate Figure 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 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.

6 Molar Mass by Freezing Point Depression Page 6 Disposal and Cleanup Your instructor will provide disposal and clean-up instructions. Graph and Calculations Graph the cooling data as shown in Figure 1. Data Tables From your graph, determine the freezing points of the solvent and each of the solutions, and the values of ΔT fp. Calculate the freezing point depression constant from the molarity and ΔT fp of the cetyl alcohol solution. Calculate the molar mass of the unknown solute using the freezing point depression constant, the mass of the unknown solute, and the mass of BHT. Enter these values in the Calculation Table. Mass of empty test tube #1, g Mass of test tube #1 plus BHT, g Mass of BHT, g Mass of weighing paper, g Mass of weighing paper plus cetyl alcohol, g Mass of cetyl alcohol, g Mass of empty test tube #2, g Mass of test tube #2 plus BHT, g Mass of BHT, g Mass of unknown, g Trial #1 Trial #2

7 Molar Mass by Freezing Point Depression Page 7 Cooling Data Table Temperature in C Time, in Seconds Pure BHT BHT + Cetyl Alcohol BHT + Unknown Calculation Table BHT BHT + Cetyl Alcohol BHT + Unknown Freezing Point, C ΔT fp, C k fp, C/m Molar mass, g/mole % error, molar mass

8 Molar Mass by Freezing Point Depression Page 8 Post-Lab Questions (Show all work on a separate sheet of paper.) 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. a. The thermometer used actually read 1.4 C too high. b. Some of the solvent was spilled before the solute was added. c. Some of the solute was spilled after it was weighed and before it was added to the solvent. d. Some of the solution was spilled after the solute and solvent were mixed but before the freezing point was determined. 2. What was the least precise measurement in the experiment? How does this limit your significant digits? 3. Did the solutions show any evidence of supercooling? 4. Why is it advantageous to choose a solvent that has a large value for K fp? 5. Explain why the pure solvent shows a level horizontal curve as solidification occurs, but the curve for the solution slopes downward slightly. Alternative Microscale Procedure Equipment 2, 6-Di-tert-butyl-4-methylphenol, BHT, 1 g Capillary tubes Cetyl alcohol, CH 3 (CH 2 ) 14 CH 2 OH, 0.1 g Cork or split rubber stopper Unknown substance, 0.1 g Balance (0.001 g precision) Beaker, 10-mL or 50-mL Small rubber band Beaker, 250-mL, or Thiele melting point tube Stirring rod Bunsen burner or hot plate, ring stand, Thermometer with 0.1 C divisions ring clamp, wire gauze with Universal clamp ceramic center Mortar and pestle or watch glass and test tube 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.

9 Molar Mass by Freezing Point Depression Page 9 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). Clamps Notched Stopper Thermometer Rubber band Notched Stopper Thermometer Clamp Capillary tube Water level Water level Rubber band Capillary tube Thiele tube Beaker Ring stand Bunsen burner Ring stand Hot plate Figure 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. Calculations Determine ΔT fp for the solution of cetyl alcohol and of the unknown substance in BHT. Calculate the molality of the cetyl alcohol solution and use it to determine the value of the freezing point depression constant, k fp, for BHT. Use the calculated value of k fp, along with the masses of the unknown solute and BHT, to find the molar mass of the unknown solute.

10 Molar Mass by Freezing Point Depression Page 10 Data Tables. Microscale Procedure Melting Points Pure BHT C BHT + cetyl alcohol C BHT + unknown C Masses BHT Cetyl Alcohol Unknown Solution #1 BHT + Cetyl Alcohol, g Solution #2 BHT + Unknown, g Calculations k fp, BHT, C/m C/m Molar mass, unknown, g/mole g/mole

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