edited by
MALCOLM M. RENFREW University of ldaho MOSCOW. ldaho 83843
Margaret J. Steffel The Ohio State University Marion Campus, Marion, OH 43302 Many general chemistry laboratory courses include an experiment far the determination of the molar weight of an "unknown" solute on the basis of its depression of the freezing point of a solvent. Such an experiment gives students an immediate and convincing demonstration of one effect that a solute has nn a solvent; also, it gives instructors a good npportunity to evaluate students'laboratory technique. A solvent that is often used is benzene (1-31. However, the Occupational Safety and Health Administration has placed benzene in its Category I of chemicals: confirmed carcinogens has6d on human data, or on tests in two mammalian species or in one species if tests have been replicated ( 4 ) . Consequently, we sought a replacement for benzene. We chose not to consider solvents that are solids a t room temperature (5)feeling that their use would be mare difficult, more time consuming, and less accurate far inexperienced workers; in addition, our stu-
Malcolm M. Rentrew draws on varied
sota, he was a supervisor of research and developmem with DuPont and General Mills; then an administrator and teacher at the University of ldaho. his Alma Mater. He is active in the American Chemical Society. including service with the Comminee on Safety and the new division of Chemical Health end Safety. He now is professor emeritus of chemisfly and is patent director of his University's ldaho Research Foundation, Inc.
dents make qualitative use of the effect that one solid has on the melting point of another in an earlier experiment involving identification of an "unknown"solid on the basis of its melting point and the melting points of mixtures. Cyclohexane is the solvent that we selected on the basis of the following criteria. Low leuel of tozicity. Benzene is considered to be a human carcinogen and is toxic through inhalation or skin absorption. Before its carcinogenicity was established it was given a health hazard rating of 2 (on a scale from 0 to 5) by the National Fire Prntection Association: Intense, continued exposure could cause temporary incapacitation or possible residual injury, unless promptly treated ( 6 ) .Its skin hazard rating is 4 (on a scale from 1tn4): Primary skin irritant-can cause severe eruptions and burns (61. The current Occupational Safety and Health Administration environmental standard for exposure to benzene is 10 ppm in air as a time-weighted average over an 8-hr period (7). A maximum environmental exposure of 1 ppm was recommended (7j, but the United States Supreme Court recently ruled that OSHA had not made the proper initial findings on which to base this standard (8,9). Cyclohexane is considered to be safer in each respect. Its health hazard rating is 1: Expasure causes irritation, but it is only minor residual injury, even if untreated (101. Itsskin hazard rating is 3: Irritant--can cause inflammation (10). The OSHA 8 ~ h rtimeweighted-average exposure standard is 300 ppm in air ( I l l . Freezing point below the lowest temperature to be expectedin the laboratory (as low as 15°C during a winter fuel shortage), so that no heating is necessary, and several degrees above 0°C so that only an ice bath is needed for cooling. The freezing point of cyclohexane is 6.6'C (121. Large enough freezing point depression constant that significant changes in freezing point can be measured for dilute solutions. The calculated value of Kf is 20.4 C" kglmole for cyclohexane (12,131in comparison to 5.13 Co kglmole for benzene (13,141. A good soluent for o variety of salutes. Although some solutes may be less soluble in cyclohexane than in benzene, cyclnhexane's large K ? m a k e possible ~ the use d m o r e dilute solutions.
benzene have similar vapor pressures of nearly 100 tom a t room temperature (12, 14). Flammability low enough that there is little danger of fire if other experiments re-
quiring burners ore being performed in the laboratory. This is a criterion by which eyelohexane is no better than benzene. Both can be ignited a t almost all temperatures. Flammability limits (volume % in air) are 1.3 to 7.1 for benzene (6) and 1.3 to8.0 for eyclahexane
grade cyclohexane is more expensive than reagent grade benzene. Cyclohexane of 99% purity can be cheaper than reagent grade benzene and might he satisfactory.
Experimental Procedure To minimize spills and inhalation of eyclohexane (as well as reduce waste) students obtain it from a buret that the instructor sets up in a hood. They measure it (to the nearest 0.01 ml) into a test tube and, while they are still a t the hood, they close the tube with a rubber stopper equipped with a stirrer and a thermometer graduated in 0.1". This tube is placed inside a larger test tube (air jacket) which is submerged in an ice bath and the freezing point of cyclohexane is measured to the nearest 0.05O on a cooling cycle. "Unknown" solutes that have been used satisfactorily are p-dichlorobenzene, biphenyl, naphthalene, and p-bromochlorobenzene, all of which have molar weights between 100 and 200 g. About 0.3 g (not more than 0.4 g) of an unknown is weighed to the nearest 0.0001 g on an analytical balance and dissolved in about 20 mlof cyclohexane.This gives solutions which have molalities of about 0.1 m to 0.2 m and exhiblt freezingpointdepressions of about 2' to4'. Incontrast, when students used benzene they prepared solutions of about 0.2 m t o 0.5 m for which the freezing point depression was about 1'-2.5' (2,3). With large classes the smaller sample size might be a financial advantage, and it is hoped that the more dilute eyclahexane solutions behave more ideally. To determine the melting point of cyclohexsne in a solution, the solution is frozen to a slurry, the test tube is placed in the air jacket, and the solution is allowed to warm to a temperature several degrees above the melting point. Temperature readings are taken a t 30-see intervals, and the students graph their data (temperature versus time) while they are in the laboratory. The intersection of the two straight-line portions of the curve (for warming while solid is present and a line of ereater slooe for warmine after all to repeat the measurements and graphing until they have temperature readings for
Volume 58
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Results
periods of a t least 3 min both above and below this point of intersection and until two values for the melting point differ hy not more than 10%. With some students, close supervision and guidance is necessary to ensure that they cool the semisolid mixture to a low enough temperature that the plot's linearity below the melting point is apparent. Because we do this experiment only with small groups of students ( I to 20 in our chemistry course for science majors) this supervision is not difficult and we require each student to have a graph approved by the instructor hefore leaving the laboratory.
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Journal of Chemical Education
T h e most recent group of students to perform this experiment reported molar weighb that ranged from 9% low to 10% high. T h e average error (ahsolute) for the ten students was 5%. and the median error was also 5%. This is comparable to, hut slightly better than. the results that ~ r e v i o u r students usually ohtained with benzene using the same equipment and procedure.
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