Molecular Mass by Freezing Point Depression
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 charge in the freezing point of a solvent when an unknown substance is dissolved in it. It has been found that the charge in a freezing is directly proportional to the molality of the solution. This change in freezing point is one of several “colligative” properties of a solution-properties that depend only on the number of dissolved particles in a solution, and not on the type of particles. 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,isdefined as moles of solute divided by kilograms of solvent:
Moles solute
m =------
Kg solvent
Since moles of solute is the same as grams solute divided by molecular mass of solute, M. then:
g solute
m=------
Kg solvent · M solute
The relation to change in freezing point is:
ΔTfp=Kfp · m
Where ΔTfp is the change in 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.
Mass
Molecular weight=------
Mole
The solvent that will be used in this experiment is a nonpolar solvent with the common name cetyl alcohol. This compound is frequently used in cosmetics as an emollient. IUPAC name for the compound is 1-Hexadecanol. Its structured formula is as follows:
CH3(CH2)14CH2OH
The freezing point of Cetyl alcohol is approximately 48oC. if the freezing point of both the solvent and the solution is determined using a thermometer which is calibrated every 0.1oC, the freezing point can be estimated in the range: ±0.01oC.
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 pure solvent freezes, or will slowly fall as the solution freezes.
Figure 1. Freezing point Graph for Pure Solvent and for Solution
Even though the melting point of cetyl alcohol is known, it is necessary to determine it with the thermometer that will be used in an experiment. Thermometers 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 thermometer reading is slightly off, the change in temperature should be accurate. It 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 Cetyl alcohol 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 Cetyl alcohol 4-(t-octyl) phenol (CH3)3CCH2C(CH3)2C6H4OH (s)
Unknown substance
Safety Alert
Organic Compounds are frequently flammable. Use caution with them. Many have offensive or toxic vapors. Work in fume hood.
Wear Chemical Splash Googles and a Chemical–Resistant apron.
Use a large test tube and a sensitive thermometer. Clamp the thermometer using a cork (not rubber) stopper which has a large hole, or spilt rubber stopper. Do not seal the test tube with cork- it is just to support the thermometer. Make a stirrer out of wire bent with a circle at the bottom. The test tube will be clamped in the beaker so the solid it contains will be well below the level of the water in the beaker. Heat the beaker with a Bunsen burner or on a hot plate.
Weight the test tube on a sensitive balance. Accurately measure about 8 grams of cetyl alcohol into the test tube using a sensitive balance. Record the weight of the test tube and the cetyl alcohol. Clamp the test tube in the water bath and insert the thermometer and stirring wire. Heat the water bath about 90oC, remove the test tube from the hot water bath and record the cetyl alcohol temperature every 20 seconds as the melted Cetyl alcohol cools. It is important to continuously stir the cetyl alcohol to maintain even cooling. Stirring will also help prevent supercooling. Continue recording values until you have at least five which are constant. Make a note of the temperature where crystals being form. If time permits repeat this measurement.
Using a sensitive balance, accurately measure about 1 gram of 4-(t-octyl) phenol onto a piece of waxed weighted paper and record its mass. Now place the 4-(t-octyl) phenol into the test tube containing cetyl alcohol. 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 first being to form. If time permits repeat this measurement.
Repeat the procedure using freshcetyl alcohol and 1 gram of the unknown compound. If time permits repeat this measurement.
Results:
1. Graph your data as in Figure 1.
2. Determine the freezing point for the solvent and the freezing point of the solutions along with the
ΔTfp using the graph.
3. Calculate the molality of the 4-(t-octyl) phenol solution and use it to calculate the freezing point
depression constant of cetyl alcohol.
4. Use the calculated value of kfp to find the molecular mass of the unknown solute.
Conclusions:
1. Draw a phase diagram of the pure substance, and show how addition of a solute affects this
diagram.
2. What is the least precise measurement? Explain. How does this limit your significant figures?
3. Why is it advantageous to choose a solvent that has a large value for kfp?
4. Explain why in Figure 1 the pure solvent shows a level horizontal curve as solidification occurs,
but the curve for the solution slopes down slightly.
Preliminary Lab Assignment:
1. The following data was obtained in an experiment designed to find the molecular mass of a solute by freezing point depression.
Solvent: para-dichlorobenzeneFreezing point depression constant: 7.1˚C/m
Freezing point of pure solvent: 53.02˚CMass of para-dichlorobenzene: 24.80 g
Mass of unknown substance: 2.04 gFreezing point of solution: 50.78˚C
Calculate the molecular mass of the solute.
2. 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 you 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 the solvent were mixed but before
the freezing point was determined.