Molecular Weight Determination by Boiling-Point Elevation of a Urea Solution Nicholas C. Thomas and Patsy Salwwan Auburn University at Montgomery, Montgomery. AL 36117 When studying the colligative properties of solutions, a common exoeriment used in eeneral chemistn, laboratories involves determining the moleeular weight of an unknown bv measurine the freezine-ooint deoression of the unknown in a naphthiene s o l u t i o ~while th'is experiment is generally straightforward to perform, the proredure is not without its problems. For instance, when heating a volatile solid such as naphthalene, it is difficult to prevent the laboratory from being filled with naphthalene fumes especially when fumehood space is limited. Throughout the course of a year, this experiment also generates a large amount of organic waste (naphthalene plus unknown, e.g., diphenyl or henzil), which is both environmentallv and economicallv undesirable. Furthermore, cleaning thk naphthalene fro& the apparatus is messv and mav reauire toxic solvents. We have overcome theseproblem8"by measuring the hoiling-point elevation of a solution of urea (the student unknown) in ethanol (normal boiling point 4 8 "C). This experiment demonstr&s the same orincioles and involves similar calculations as the freezing-point lowering experiment but avoids the unwanted problems outlined above. Although the boiling-point elevation constant for ethanol is low (Kb = 1.22 "Clm) compared t o the freezing-point depression constant of naphthalene (Kf= 6.85 OCIm), we have found the molecular weight determination for urea to be accurate and precise. Using the simple apparatus shown in the figure, no solvent fumes are produced since the l e n a h of elass tubine serves as an adea& condenser to Drevent so6ent loss th;ough evaporation:~heuse of a capill&y tube results in a steadv stream of bubbles beine oroduced. and so the boiling pointmay easily be measured- For a solution containing 2.50 g of urea per 25.00 mL of ethanol (equivalent to a molality of 2.12 m), we consistently observed boilinp-point elevations between 2.3 and 2.7 OC using accurate thermometers with0.1 "C suhdivisions. From these data, molecular weights ranging from 67.8 to 57.7 were obtained (the actual molecular weight for urea is 60.1). The use of accurate thermometers is recommended since this yields a fairly accurate measurement of the molecular weight. Consequently this is a good experiment t o evaluate students'auantitative laboratorvskills. At the ehd of the experimentstudents may pour the hot urea solution into a large beaker where the solute will immediately begin to recrystallize on cooling. When all students have disposed of the urea solutions and crystallization is complete, the urea may be recovered simply by filtration. It mav then he stored and reused for future classes. Since urea is very soluble in water, glassware still contaminated with traces of urea may be easily cleaned by rinsing under a tap. Experlrnental Caution: Ethanol is flammable, keep away from flames. If available, stirring hot plates may be used to heat the water baths. Prepare first a 90 'C water hath by heating 400 mL of water in a BOO-mL beaker. To a clean, dry 6- X 1-in. test tube add 26.00 mL of reagent ethanol using a volumetric pipet. A small capillary tube (about 3-4
in. long) is placed in the ethanol, open end down. The test tube is then fitted with a two-holed no. 4 rubber stopper with a thermometer inserted so it is about an inch below the surface of the ethanol, and an 8-in. length of glass tubing. (Insertion of the thermometer and glass tubing should be done by stockroom personnel.) A thermometer with 0.1 *C subdivisions should be used for the most accurate result. However, standard 110 O C thermometers with 1OC subdivisions may be used hut give a less accurate molecular weight determination. The test tube is clamped and immersed in the water bath (see figure) to a depth of about 1 in. When the ethanol begins to boil gently, as indicated by a steady stream of bubbles from the capillary tube, the temperature is read to the nearest 0.1 OC for several minutes until a constant value is obtained. This is the boiling point of the ethanol. Vigorous boiling should he avoided since this can result in loss of solvent through the glass tubing. Raise the test tube out of the water hath. and allow the tube to cool a minute. Cnrefullv remove the stopper, and add about 2.5 g of the unknown.and recurd the actual mass used. Replace the stopper, and place the test cube back in the water hath. Alluw the solution to hoil gently for several ~~
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Apparatus fadetsrmlnatlon of bolllng point. (Mcdlfled horn Scalle. C. W. J.: bachley. 0.T. Ctmmishy in h h b w a t o r y : Saunders: PA. 1987: p 258). Volume 67 Number 11 November 1990
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minutes until all the solure has d i ~ s d v ~and d . w o r d the boiling point of therolutim. Discard the hot erhanolrolurion in the beaker provided. Rinse the test tube and thermometer under a tap to clean. Alternative student unknowns which have a range of molecular weights include tartaric, benzoic, salicylic, and laurie acids. Since some of these do not dissolve as quickly as urea, a wire stirrer may be inserted through the glass tube toaid dissolution. Alternatively, if a stirrine hot nlateis used. asmall stirrine bar should be ~laeedin the test tcbe. Stirring should be discontiked once the solid has dissolved.
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Journal of Chemlcal Education
Calculations Students first determine t h e rnolality of t h e solution from the measured boiling-point elevation and t h e K~ for ethanol (1.22 'C/m). Given the density of ethanol (0.785g/cm3), the mass of ethanol used mav be found and hence t h e mass of unknown in 1kg of solvent. From these data and the molality, the molecular weight may be determined.
