Molecular Weight
o f Volatile
Liquids by Freezing Point Depression
Molecular weight by freezing point depression is a common experiment in freshman them- Summary of Determination of Molecular Weights of Volatile istry laboratory. The theory as presented in textSolutes by Freezing Point Depression Method hwks and laboratory manuals is usually dis~rpericussed in terms of nan-volatile solutes. As Katzl recently pointed out the restriction to non-volaAverage Number tile salutes is unnecessary and is used as a conveMoleevlar Molecular Average of solute nience to the authors for their discussions of the Weight Weight Deviation Samplings theory. Laboratory manuals invariably reeomXylene 15 mend solids for use as unknowns. I t is unfortu- Chlom~orm 16 "ate that students and faculty alike are left with ~itmbenzene 19 Carbon Tetrachloride 18 the impression that the theory is applicable only Chlorobenzene 10 to the ease of non-volatile solutes. Acetone 7 In a freezing point depression experiment in ~ ~ t ~ h ~ d ~ f ~ ~ ~ ~ our freshman chemistw lahoratorv. we have suc- Toluene cessfully used the voiatile liquid; listed in the Dioxane 88 91 15 17 88 90 10 15 table as the unknown solutes with benzene as the 72 75 16 20 solvent. Presented for each unknown is the average molecular weight of a large number of samplings along with the average deviation. The results reported here are comparable to results we have obtained using solid solutes under the same experimental conditions. The major contribution to the average deviations is not the volatility of the liquid, hut rather due to the inherent error in the determination of (1) the freezing point of pure benzene, (2) the freezing paint of the solution, and (3) the weighing of solute and solvent. Our freshman students are presently using thermometers which are graduated in one-half degrees. The percent error involved in determining AT with these thermometers is in the range 6-15%. A much smaller error is involved in weighing. Any error introduced during the experiment due to the volatility of the solute is minimized by the weighing procedure employed. All weighings are made directly in the test tube in which the freezing points of the liquids are to be determined. Students are instructed to make 3 weighings: (1) test tube empty, (2) test tube with benzene and (3) test tube with benzene plus solute. The volatile solutes are introduced by adding a specified number of drops. The test tube should he stoppered during the weighing process and maintained in a vertical position by placing in a small bea-
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All of rhr materials listed arc relatively rnrrpenswc in comparison to the suhdr recummended for u e uith henrent- I t rhould he pointed out that not all liquids give satisfacrory rcsulrs. For examplr, prupanul, butanul, pentand, and a c e t ~ ar o d z i w erroneous results due to the nun-ldeallts of them benzene ralutioni. In genera1,iiquids have an advantage over solids in that they are more readily soluble in the solvent, and are easily dispensed by means of dmpper bottles. Use of liquids as well as solids as unknowns gives the instructor greater flexibility in the selection of molecular weights since many common liquids have lower molecular weights than the solids which are traditionally used and will thus eliminate the impression that the theory applies only to non-volatile salutes. 'Katz, L., J . CHEM.EDUC.,44,282 (1967) Richard C. Parker David S. Kristol Newark College of Engineering 323 High Street Newark, New Jersey 07102
492 1 Journal of Chemical Education
