AP Chem                                                                                             Unit 10-11

Chapter 10-11                                                                                     Liquids and Solids

Lab 10

 

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

 

There are a number of ways of determining the molecular 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. It has been found that 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 a solution, and not on the type of particle. Other colligative properties include change in boiling point, vapor pressure and osmotic pressure. Measurements of these properties can also be used to find molecular mass of solutes.

 

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

 

            m = moles solute

                                Kg solvent

 

Since moles of solute is the same as grams solute divided by molecular mass of solute, M, then:

 

                        m =  g solute    

                                kg solvent x M solute

 

The relation to change in freezing point is:

 

                        ΔTfp = kfpm

 

            Where ΔTfp is the change in the freezing point, kfp is the freezing point depression constant for the solvent, and m is the molality of the solution. The value of kfp must be determined for each solvent.

 

The equations are combined as follows:

                        M solute = kfp x g solute

                                          Kg solvent x ΔTfp

 

The solvent that will be 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 as follows:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


The freezing point of BHT is approximately 70°C. If the freezing point of both the solvent and the solution is determined using a thermometer which is calibrated every 0.1°C, the freezing point can be estimated in the range: + 0.01°C.

Figure 1 shows a cooling curve for a pure solvent and for a solution. Notice that supercooling may occur in both the solvent and the solution. If it does, as the crystals begin to form the temperature will rise slightly and then remain constant as the pure solvent freezes, or will slowly fall as the solution freezes.

 

Temperature
 

pure
 

supercooling
 

solution
 
 

 

 

 

 

 

 

 

 

 

 

 

 

Time of cooling
 
 

 

 


(Figure 1-Freezing Point Graph for Pure solvent and Solution systems)

 

 

Even though the melting point of butylated hydroxytoluene is known, it is necessary to determine it with the temperature probe that will be used in the experiment. Probes can give temperature readings that are slightly different from true values. In this experiment, we will be using the change in temperature to calculate the molecular mass. Even if the probe 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 of the solvent and that of the solution.

 

It is also necessary to measure the change in freezing point using a known solute in the BHT so that the freezing point depression constant can be calculated. Once this is known, the molecular mass of an unknown substance can be determined.

 

Chemicals:

            Butylated hydroxytoluene, BHT, C15H24O (s)

            Nathalene, C10H8 (s)

            Unknown substance

 

Equipment:

            Test tube, 18- x 150-mm

            Beaker, large

            Universal clamps

            Cork or split rubber stopper with one hole

            Balance, sensitive

            Temperature probe (use Logger Pro 3.4.1)

            Ring stand

            Wire stirrer

            Hot plate, utility clamps

            Weighing paper

 

Procedure:

Safety Alert

Stirring wire
 
            Organic compounds are frequently flammable. Use caution with them. Many have offensive or toxic vapors. Work in a fume hood. Wear chemical splash goggles and a chemical-resistant apron.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


(Figure 2 Diagram of Freezing Point Apparatus)

Part 1

Assemble an apparatus like the one diagrammed in Figure 2. Use a large test tube and a probe. Clamp the probe using a utility clamp and paper towel. Clamp the tube with a second utility clamp. Make a stirrer out of wire bent with a circle at the bottom. The test tube will be clamped in the beaker so that the solid it contains will be well below the level of the water in the beaker. Heat the beaker/water bath on a hot plate.

 

Start Logger Pro and use Experiment/Connect Interface/LabPro. Also go to Experiment/Data Collection/Triggering/Collection/300 sec (not 180 sec). When the data set is completed, it will have automatically created a graph but you may have to rescale its axis by double clicking on them and use Manual for scale increments to create a full scale graph.

 

Weigh the test tube on a sensitive balance. Accurately measure about 8 grams of BHT into the test tube using a sensitive balance. Record the weight of the test tube and the BHT. Clamp the test tube in the water bath and insert the probe and stirring wire. Heat the water bath to about 90°C, remove the test tube from the hot water bath and start recording data by pressing the collect button in Logger Pro. The system will record the BHT temperature every  second for 300 sec as the melted BHT cools. It is important to continuously stir the BHT to maintain even cooling. Stirring will also help prevent supercooling. Continue recording values until you have at least five that are constant. Make a note of the temperature where crystals begin to form. If time permits, repeat this measurement.

Part 2

Using a sensitive balance, accurately measure about 1 gram of  napthalene onto a piece of waxed weighing paper and record its mass. Now place the napthalene into the test tube containing BHT. Heat the mixture in the hot water bath until the substances are all melted. Stir well to be sure that the solution is homogeneous. Again remove the test tube from the hot water bath, stir, and record temperatures every 20 seconds until you have at least six values after crystals begin to form. If time permits repeat this measurement.

Part 3

Repeat this procedure using fresh BHT and about 1 gram of the unknown compound. If time permits, repeat this measurement.

 

Disposal and Cleanup:

Place the test tubes in a boiling hot water bath (with soap)until the mixtures are melted. Pour the melted substance out onto crumpled newspaper or paper towels. Consult the Flinn Chemical Catalog/Reference Manual, suggested disposal method #26a for disposal instructions. Rinse test tubes with acetone or ethanol before using detergent and water to clean them.

 

Graph and Calculations:

Graph your data as in Figure 1. Determine the freezing point of the solvent and each of the solutions, and the values of ΔTfp from your graph. Calculate the freezing point depression constant from the data using napthalene. Then calculate the molecular mass of the unknown solute. Determine ΔTfp for the solution of napthalene and of the unknown substance in BHT. Calculate the molality of the napthalene solution and use it to calculate the value of the freezing point depression constant, kfp, for BHT. Then use the freezing constant to determine the mass of the unknown sample.

 

 

Analysis Questions:

  1. Give a definition of colligative properties.
  2. Draw a phase diagram of a pure substance, and show how addition of a solute affects this diagram.
  3. What is the least precise measurement? How does this limit your significant digits?
  4. Why is it advantageous to choose a solvent that has a large value for kfp?
  5. Explain why the pure solvent shows a level horizontal curve as solidification occurs, but the curve for the solution slopes downward slightly.
  6. The following data was obtained in an experiment designed to find the molecular mass of a solute by freezing point depression. Calculate the molecular mass of the solute.

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

 

 

 

 

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

 

 

 

 

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

 

 

 

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