55 Experiment #4. Molecular Weight by Freezing Point Depression 4. MOLECULAR WEIGHT BY FREEZING POINT DEPRESSION In this experiment you will use a colligative property to determine the molecular weight of an unknown substance by freezing point measurements in cyclohexane. Colligative properties of solutions are properties that depend on the collection of particles present, that is, on the number of particles, rather than on the kind. Freezing point depression is a colligative property and may be defined as the following: for a given solvent the freezing point lowering is directly proportional to the concentration of particles dissolved in it. Tf = Kfpm Where Tf is the change in freezing point, Kfp is the molal freezing point constant, and m is the molality of the solution (moles of solute/kg of solvent). The normal freezing point of cyclohexane is 6.6 C and the Kfp for cyclohexane is -20.4 C kg/mole. Cooling Curve of a Pure Substance and of a Solution
56 Experiment #4. Molecular Weight by Freezing Point Depression Experimental Procedure A 25 x 100 mm test tube is fitted with a 2-hole stopper and a 0.1 C thermometer or temperature probe is inserted into one of the holes. (CAUTION: Be very careful if you are inserting thermometer into the rubber stopper. Use a twisting motion on the stopper while holding the thermometer steady with your hand next to the stopper). A stirrer is made from copper wire and is inserted into the other hole of the stopper. Weigh the empty test tube to the nearest 0.01 g on the top load balance. Add about 5 cm of cyclohexane and weigh the tube and cyclohexane. (It may be easier to place the test tube in a 150 ml beaker during these weighings). Assemble the apparatus and determine the freezing point of the pure cyclohexane. The rate of cooling may be controlled by the depth the test tube is immersed in the ice bath. The temperature from the time of immersion should be recorded every 30 seconds to the nearest 0.1 C. It may also be helpful to note the temperature at which crystals begin to form on the side of the tube. Now that you have a good idea at what temperature cyclohexane freezes, repeat the freezing process but this time use an outer jacket on the test tube with the cyclohexane in it. A 38 x 50 mm test tube may be used as a jacket and the
57 Experiment #4. Molecular Weight by Freezing Point Depression cyclohexane in the first test tube should be cooled to approximately 1.5-2.0 C above the suspected freezing point. Now enclose the first test tube in the outer jacket and take time versus temperature readings until you are certain the cyclohexane is frozen. The temperature drifts will be much slower now due to the outer jacket being used. After the run is complete, raise the apparatus from the bath, remove the outer jacket, and warm the cyclohexane gently several degrees with the warmth from your hand until the solid has melted. Now replace the jacket and repeat the freezing process, so that the freezing point can be determined as accurately as possible. Raise the stopper and thermometer and add 0.3-0.4 g (±0.0001 g) of solid unknown to the weighed amount of cyclohexane. Do not lose drops of solvent in this operation. Reassemble the apparatus and stir the mixture until the all of the solid is completely dissolved. You may have to warm the mixture slightly. Determine the freezing point of this solution without using the outer jacket. Warm the solution up several degrees above the suspected freezing point, enclose the tube in the outer jacket and repeat the freezing process taking readings for 15-20 minutes. Be sure your ice bath is very cold at the beginning of a run (i.e. 0 o C or less). When you are finished the experiment pour your used cyclohexane mixtures into the return bottles which are located in the fume hoods. For all runs, plot the time-temperature data readings on graph paper. From the graphs the freezing points can be determined very accurately and the molecular weight of the unknown solute can be calculated. Be sure to include in your report the number of the unknown sample being used.
58 Experiment #4. Molecular Weight by Freezing Point Depression Molecular Weight by Freezing Point Depression DATA & REPORT SHEET: Name: Partner Credit: Y (Circle if yes) Partner's Name: _ Unknown Sample # Lab Section: Mass of test tube + beaker + cyclohexane = Mass of test tube + beaker = Mass of cyclohexane used = Mass of weighing paper + unknown solute = Mass of weighing paper = Mass of unknown solute used = Trial #1 Trial #2 Trial #3 Trial #4 Time Temp o C Time Temp o C Time Temp o C Time Temp o C
59 Experiment #4. Molecular Weight by Freezing Point Depression Trial #1 Trial #2 Trial #3 Trial #4 Time Temp o C Time Temp o C Time Temp o C Time Temp o C
60 Experiment #4. Molecular Weight by Freezing Point Depression Molecular Weight by Freezing Point Depression REPORT SHEET: Name: Partner Credit: Y (Circle if yes) Partner's Name: _ Lab Section: Unknown Sample # Mass of pure solvent (cyclohexane) : _ Freezing point of pure solvent : Trial # 1. (without jacket) Trial # 2. (with jacket) Freezing point of the Solution: Mass of unknown solute: Trial # 3. (without jacket) Trial # 4. (with jacket) Calculations: T f = (without jacket) T f = (with jacket) K f = o C kg / mol #1. Calculated m value = ( units) (without jacket) #2. Calculated m value = ( units) (with jacket) Molecular weight of unknown sample # using m value #1. = _ using m value #2. = _
61 Experiment #4. Molecular Weight by Freezing Point Depression Molecular Weight by Freezing Point Depression: Pre-Lab Questions 1. Define the term T f. 2. What pure solvent will be used in this experiment? 3. Which should be lower, the freezing point of solvent A plus solute B or the freezing point of pure solvent A? 4. Name one colligative property of solutions. 5. Where should you pour your used cyclohexane solution when you are finished the experiment? 6. How can you prepare an ice bath with a temperature lower than 0 o C? 7. Define the term K fp. 8. Define the term molality. 9. Given that the freezing point of 16.55 g of cyclohexane is measured as 6.65 o C and the freezing point of the pure cyclohexane plus 0.3559 g of an unknown solute is 4.67 o C, what is the molecular weight of the unknown solute?