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MOLECULAR WEIGHT DETERMINATION BY BOILING-POINT ELEVATION A Freshman Research Project' ENNO WOLTHUIS, MARILYN VISSER, and IRENE OPPENHUIZEN Calvin College, Grand Rapids, Michigan
INTEREBTED and q u a e d students must be challenged early in their college careers to do more than read textbooks, memorizelecture notes, and repeat in the laboratory what many others have done before them and in the same way. Therefore i t has been our policy to assign to such students, usually majors in chemistry or chemical engineering, a research project on which they spend approximately one-half of their laboratory time during the second semester of the first year in general chemistry. Up t o this time our emphasis in this program has been on a critical examination and improvement of laboratory experiments, primarily those of a quantitative nature. Each project has required of the student a careful examination of the literature, the keeping of an accurate record of all experimental data, and the writing of a complete, wellorganized report on the entire project, altogether constituting an invaluable experience. The results of one successful project were reported previou~ly.~This paper presents the work done on another project which culminated in a reliable laboratory experiment. Only a very few laboratory manuals include an experiment on the determination of molecular weight by boiling-point elevation, and the periodical literature contains little more which can be used in the general chemistry laboratory program. Rose and Billingera described a simple apparatus and method which they used successfully with a molal solution of naphthalene in carbon tetrachloride. They reported no results obtained with other solutes or with other solution concentrations. It has been our experience that their recommendation for a hath temperature 10 degrees above that of the solvent or solution tends t o give erratic results, apparently because of too vigorous boiling and loss of solvent. Hoyt4 has described a fairly simple Cottrell pump for the measurement of boiling points, which was reported t o give good molecular weight results with naphthalene, camphor, pdichlorobenzene, p-dibromobenzene, and benzoic amd, using 3 g. of each in 15 ml. carbon tetrachloride. 1 This paper reports the work done by Marilyn Visser and Irene Oppenhuizen as part of their freshman year's work a t Calvin College. ' WOLTHUIB,E., D. DEVRIES, AND M. POUTSMA,J. CAEM. Eouc., 34, 133-34 (1957). J. CHEM.EDUC.,7, 2715-18 ROSE,A., AND R. D. BILLINGEE, (1930). 'Horn, C. S., J. CHEM.EDUC.,24, 563-64 (1947).
I & IN
We set out to study the factors which affect the accuracy of the molecular weight results. I n order t o keep the apparatus as simple as possible for general laboratory work, we chose to use the general method of Rose and Billinger as a starting point in our investigations. After studying a number of variables, d e scribed below, the following procedure was evolved and proved to be reliable by actual class test. PROCEDURE Fit an 8-in. test tuhe with a twwhole rubber stopper, one hole for a thermometer, which can be read to two-tenths of a degree C., extending to 'I1 in. above the bottom center of the test tube, and the other hale for a l-ft. length of glass tubing to serve as a condenser, with its lower end just protruding through the bottom of the stopper. Set a 600-ml. beaker on a. wire gauze and ring attached to a stand a t a height convenient for heating with a burner. Also attach the test tube assembly to the ring stand so that the bottom of the test tube is in the center and about 1in. from the bottom of the beaker. Suspend a one-degree thermometer in the water hath such that its lower end is 2 in. ahove the bottom of the beaker and touching the test tuhe. The entire assembly should appear as in the figure. Remove the stopper and oarefully pour into the test tube 20 ml. carbon tetrachloride (density = 1.59 g./cc.) measured as JOURNAL OF CHEMICAL EDUCATION
accurately as possible with a 25-ml. graduated cylinder. Add a boiling stone (porous clay plate), and replace the stopper, taking care that the bulb of the thermometer is in the center of the test tube. (Wehave found a. boiling stone more effeetive than either glass beads or capillary tubes.) Nearly fill the beaker with water and heat it with stirring t o 81% Continue t o stir the water while adjusting the burner to maintain a bath temperature of 81"-82°C. Addition of a little cold water is an effective r a y to keep the temperature from rising above 82°C. The carbon tetrachloride should now be boiling gently and its temperature should be fairly constant. Record this temperature each minute for the next five minutes while keeping the bath temperature 4' lo 6' above that oflhe solvent. Be sure to stir the water continuously during this time. The five readings should not vary more than 0.2", and the average of these is recorded as the boiling point of the solvent,. Stop heating, and raise the test tube out of the water bath, While the apparatus is cooling, obtain from the instructor an envelope containing an unknown solid whose molecular weight must be determined, and weigh the envelope and contents on an analytical balance to the nearest centigram. Tear open one end of the envelope, and very carefully pour its contents through s. powder funnel into the test tube containing tho solvent. At once replace the stopper and shake the tube gently to dissolve the solid solute. Clamp the test tube so that i t is suspended in the bath ezoetly as befove. Weigh the empty envelope to obtain the weight of the unknown salute. Momentarily lift up the stopper to add another boiling stone and replsce it firmly a t once. Heat the water bath again, this time t o about 85"-88'C. Watch both thermometers and adjust the bath temperature t o 4'-5" above that of the solution, stirring the bath all the while. When the adjustment has been made, the solution should be boiling gently. Then record the temperature of the solution each minute for the next five minutes. These readings, too, should not vary more than 0.2", and their average is recorded as the boiling point of the solution. Calculate the molecular weight of the unknown solid. If the percentage composition is given, also calculate its molecular formula.
several readings with the bath temperature 4"-5" above that of the solvent, data were obtained t o show that when the hath temperature was 85.5"C. (3.2' above that of the boiling solution) the boiling-point elevation was 5.2" to give a calculated solute molecular weight of 243. As the bath temperature rose t o 88.1°C. (5.5" above that of the boiling solution) the boilingpoint elevation was 5.5', to give a calculated solute molecular weight of 230. Similar data also were obtained using solutions of naphthalene, p-dichlorobenzene, and biphenyl. These data lead to the following conclusions: (1) The observed, or calculated, molecular weight decreases with an increase in the difference between bath and solution temperaturee. (2) The most accurate results are obtained when the bath temperature is 4'-5O above that of the solution. (3) There is no need to define this temperature differential t o less than a degree since the corresponding variation in hoiling-point elevation is no more than 0.l0, and the reading of the thermometer can be no more accurate than t o 0.1". I n this connection it should be pointed out, also to the student, that, as a chain is no stronger than its weakest link, this molecular weight determination is no more accurate than to 14-5 units. Concentration of Solution. No attempt was made to use solutions less than one molal in strength since a t least a 5degree elevation is desirable for a reasonable degree of accuracy. However, the concentration was increased to 2 molal, with the results as shown in the table.
VARIABLES STUDIED
A variety of possible solvents were considered, but carbon tetrachloride was soon chosen as the most generally useful because of its excellent solvent properties, its relatively high ebullioscopic constant (5.0 degrees for a 1.0 mold solution), and its noninflammability. Distillation of the carbon tetrachloride was carried out to make sure it was sufficiently pure. Ninety per cent of it was found to have a boiling range of no more than 0.5"C. Bath Temperature. It is common knowledge that the observed boiling point of a liquid is affected appreciably by variations in the temperature of the bath. Since this factor is very important in this experiment and one which must be carefully defined for the student experimenter, a number of experiments were run t o determine the proper temperature differential between bath and solution. First it was found that the boiling point of carbon tetrachloride alone varied from 76.6' to 78' as the hath temperature increased from 78" to 84.5". I t was also observed that the condensing vapors rose t o the top of the test tube with a bath temperature 4"-5' above that of the boiling solvent. I t was obvious, therefore, that no more than a 5' differential is necessary or desirable. In order to define this value still more accurately, a 1.07 molal solution of pdibromobenzene (molecular weight 236) was used, and boiling-point readings were taken each minute while the bath temperature was gradually raised during a 15-minute period. Taking the boiling point of pure CC14 as 77.1°, the average of VOLUME 35, NO. 8, AUGUST, 1958
Effect of Solution Concentration o n Observed Molecular Weight Molality of s o h
pDichloro- p-Dibromobenzene benzene Biphenyl
Naphthalene
1.
154 151 151
239 234 239
157 161 161
124 131 134
1.5
154 158
247 250
173 176
144 141
2.
157 160
260 263
171 178
146 142
147
236
154
128
True mol. wt.
.
These data indicate that generally best results are obtained with an approximately molal solution, the observed molecular weight increa~ingwith molality as might he anticipated. EXPERIMENTAL RESULTS
Each member of a class of 14 freshman chemistry students was given an envelope containing approximately 8 g. of p-dibromobenxene, but its identity was witheld. Employing the procedure outlined above, the following results were obtained. Average molecular weight. . . . . 240 Maximum . . . . . . . . . . . . . . . . . . . 266 . . 221 Minimum.. . . . . . . . . . ... Average deviation.. . . . . . . . . . .
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One-half of these students obtained a value within 6 units of the true molecular weight. I n order t o compare this method for determining the
molecular weight with the usual freezing-point depression method, another sample of the same unknown was subjected to the latter procedure by the same students during the same laboratory period. Using benzene as solvent, the average molecular weight was 243, and the distribution of results was about the same as above,
indicating that the reliability of both methods is about the same. A laboratory experiment requiring the application of both methods in determining the molecular weight of an unknown solute sample can be performed conveniently in a 3-4 hour period.
