188
JOURNAL OF
CHEMICAL EDUCATION
A LABORATORY EXERCISE IN FREEZING POINT DEPRESSION WITH CAMPHOR AS THE SOLVENT E. L. GUNN
LABORATORY manuals of general chemistry often include an exercise which applies the cryoscopic or freezing point depression principle in the determination of molecular weights. The procedure usually requires the use of special apparatus with expensive thermometers where such solvents as benzene, water, and so forth are used. For these and other reasons the disadvantages of the cryoscopic method often make its use as an experiment for the individual student almost prohibitive. The writer has attempted to devise an exercise which would circumvent many of these di5iculties. In so doing, a degree of compromise in the accuracy of results had of necessity to be made. In the opinion of the writer the advantages gained in simplicity of technic and design justify the sacrifice in accuracy of results. The cryoscopic constant for camphor is given by some handbooks to he 49.8.' Thus, a mol of solute dissolved in camphor will produce a depression of the freezing point which is around nine times as great as that produced by a mol of solute in benzene. This large constant, in effect, magnifies the freezing point depression for a solute in camphor to such an extent that an ordinary thermometer with a range of 200°C. can he used. The advantage of this has been recognized in the use of camphor as a solvent for micro molecular weight determinati~ns.~However, as far as the writer knows, no experiment employing this method for the use of general chemistry students has been described. The following gives an exercise in which salicylic acid is the solute and camphor the solvent. The approximate molecular weight of the solute may be calculated from the data obtained. The freezing point of the tamphor used is determined. -
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"Van Nostrand's chemical annual," 7th issue, D. Van Nostrand Co. Inc., New York City, 1934, p. 816. Rnsr. Ber., 55,1051,3727 (1922).
Lee Junior College, Goose Creek, Texas
From one hundred to two hundred milligrams of cylic acid are weighed into an ordinary five-inch tube. The weight of the camphor used may conveniently be from twelve to fifteen times as great and should he weighed to the centigram. The camphor is easily handled when cut into small shavings. The tube is stoppered with cork, placed into a bath a t a temperature of 160'-17O0C., and the mixture melted rapidly in this temperature range. The bath used is a beaker containing 350400 ml. of cottonseed oil. The tube and contents should be kept well submerged in the bath. The temperature of the bath is allowed to fall gradually, while the test tube is gently agitated in proximity to the bulb of the thermometer, until small white specks appear throughout the solution in the tube. The temperature is immediately read. The freezing point of the mixture may he rechecked and an average taken. Each student may make three or more determinations. Experience with this procedure has shown that around fifty per cent of the determinations made will be in error by less than five per cent from the accepted molecular weight of salicylic acid, 138. The following are some of the more obvious sources of error. If the walls of the test tube are so thick as to interfere seriously with rapid heat transfer the observed freezing point will be in error. Slow, cautious cooling will minimize this. Camphor tends to suhlimate rrtther rapidly near its melting point. Keeping the tube well submerged in the bath, with rapid melting, will lessen the amount of sublimation. Inaccuracies in the weight of the solute or of the solvent obviously influence the quality of the,results. Many instructors would prefer to issue tubes containing the weighed solute to the students. for this reason. The writer wishes to thank his colleague, Dr. W. C. Thompson, for his suggestions in adapting this experiment to student use.
