Jeff C. Davis, Jr. University of South Florida
Tampa, Florida
Acetamide as a Solvent for Freezing Point Depression and Solubility Experiments
Nearly every laboratory manual in general and physical chemistry includes a freezing point depression experiment for the determination of molecular weights. With simple equipment reasonably accurate results can be obtained, even in an elementary laboratory, providing mass can be measured to 1 mg and temperature can be estimated to a tenth of a degree or better. Naphthalene has been used widely as a solvent because its melting point is attained conveniently by a hot water bath. I t is suitable, however, only for nonionic solutes. Water, while a useful solvent for both covalent and ionic solutes, is less satisfactory as far as a simple experimental procedure and materials are concerned. The solvent properties of acetamide have been known for many years.' Many nonionic and ionic solids are soluble in acetamide and its melting point, around 7g°C, suggests its use as a solvent for molecular weight experiments. Any of several common setups involving a 25-mm od test tube, stoppered with a thermometer and a simple loop agitator, and a beaker water bath are satisfactory. The cooling curve of acetamide characteristically exhibits a short supercooling period and a well-defined freezing point plateau.
Samples containing 20-30 g of acetamide and 0.5-1.0 g of solute permit the use of several ionic solids which are generally less soluble than most of the useful covalent solutes. Among the solutes which have been used successfully are naphthalene, biphenyl, camphor, urea, potassium iodide, potassium bromide, sodium iodide, ammonium iodide, and aluminum chloride. With the choice of several nonionic as well as ionic solutes the same solvent can be used to establish a molal freezing point constant (approximately 3.S°C), determine the molecular weight of an unknown, and illustrate the abnormal freezing point depression by ionic solutes. Students in high school classes, high school teachers, and beginning college classes have performed these measurements successfully. Although the melting point of acetarnide varies with the purity and source, very consistent results can he obtained within a single class using the same materials and method. Acetamide can serve also as a solvent for research-oriented experiments such as the correlation of solubilities with molecular weights, ionic radii and charges, lattice energies, and other parameters. Acknowledgment
Appreciation is extended to Drs. Paul Westmeyer and
' STAFFORD, F. O., J. Am. Chem. Sac., 55, 3987 (1933).
ZD~v~ J. sC,.,JR., AND WESTMEYER, P., J. CHEM.EDUC,43, 103 (1966). Grant GE-4699, In-Service Institute in Chemistry for Secondary School Teachers, 1964-5.
J. J. Lagowski for helpful suggestions, to the participants of the San Antonio School Chemistry Project2 for trying many variations in procedure and materials, and to the National Science Foundation for partial support of the study.a
Volume 43, Number 1 1 , November 1966
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