An Addendum to: A Simultaneous Analysis Problem for Advanced General Chemistry Laboratories A recent note in THIS JOURNAL' described a minor madification in the laboratory procedure used in the very traditional experiment for the determination of the empirical formula of MgO by burning magnesium metal in air.2 Briefly, when magnesium burns in air two competing reactions mcur:
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Mg + %02 Mg + %NI
MgO
%Mg3Nz
The MgsNz is converted to MgO by adding water and reheating. The conversion of magnesium nitride to magnesium oxide produces a small increase in mass. The experimental goal of the note was to illustrate how the percentage of MgsN2produced in the initial reaction could be calculated if only one additional weiehine was nerformed. The experiment has been successfully performed by many classes of students a t JMU. Analytical balances have been routinely used because the change in mass being determined is very small, on the order of a few milligrams. In a recent class, a surprisingly large number of students observed a weight loss when a weight gain was anticipated and a careful study was undertaken to determine the source of this anomalous behavior. Initially, problems associated with laboratory technique were suspected. Experiments were performed that systematically eliminated the following sources of error: the balances, the dessicant, the temperature of the crucibles when they were weighed, the pretreatment of the crucibles and the completeness af the reaction of MgsN2 with water. During the course of these studies two obsenations suggested that the source of the ~roblemmight involve additional comnetinn chemical reactions and not laboraton teehnioue. First. the dark-colored material, oriainaliv thouehtto be onlv. M~>N?. with water andhid not &mnke , ~~-~ ,. .. nnon . - .aatenrlerl ~~ -. dia not react eomoletelv heating. ~ e c u n dthe . amount of the persistent dark mateial appeared to be a function of the heat source. IMekrr hurners appeared to produce more of the dark material than did nunsen burnrrs, while vinuallv none utthis mawrial was prudured if the reactions were run in a muffle furnace.) A brief review of the chemistry of magnesium revealed that over a half century ago it was reported in a monograph on magnesium that this metal, at elevated temperatures, can reduce the oxides of carbon to the element.3 If elemental carbon is produced in the initial reaction with magnesium, hut then oxidized in the second heating, the observed loss of weight could be accounted for. An experimental confirmation of the presence of carbon was performed by running the entire experiment according to the procedure published earlier,' transferring the contents of the crucible into a reaction boat, placing the boat in a tube furnace, and heating in a stream of pure oxygen at approximately 500'C. The effluent was bubbled through a saturated barium hydroxide solution. The appearance of a white precipitate indicated the presence of BaC03, which was confirmed by complete reaction with HCI. Simultaneously, the dark material in the MgOIMgaNz reaction mixture disappeared, thus verifying the presence of carbon. The amount of elemental carbon produced in the initial reaction is extremely variable. Magnesium oxide, magnesium nitride mixtures with a mass of approximately 400 mg contained up to 12 mg of carbon depending upon the heat source. These results were not entirely surprising because of the simificant concentration of CO? in the atmosnhere of a burner flame. The prwlous rrsults provide a simple explanation fur the anomnlwu hehavior that war occasionally ohserved in the experlmrnt described earlier.' l i a flame is used as the heat wuwe, pruhlems awxiated with the reduction or the oudes of carbon can berignificantly reduced iithecrucihleix tiltedxo that theopening hetween therruc~bieanditscop isnor en~
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In the original note on the determination of the percent Mg3N2,the authors noted that the experiment was not intended to be a quantitative analysis experiment, but rather a demonstration of the power of the mole concept to simplify difficult stoichiometrv Droblems. The authors maintain their orieinal m i t i o n but think that this corresnondence is necessarv because there is a si&if;cant difference between an experiment t&b;definition is not quantitative, andone thatoc~asion&~rovides results that are apparently impossible.
' Leary. J. J., and Gallaher. T. N.. J. CHEM.EDUC.. 60, 673 (1983).
a) Dingledy. 0. P.. "Synthesis and Determination of the Formula ofa Compound." STQ1-168 Modular Laboratory Program in Chamislry. c 3rd ed., Willard Grant Press, Boston. 197%b) Sherman, A,, Sherman. S., and Russikofl. L.. "Labaratory Expriments for ~ a s i ~hemislry." Houghton-Miffiin Company. Boston. 1984. '"Magnesium." American Magnesium Corpwation, Niagsra Fails, NY, 1923, p. 9. 'AUlhor to whom correspondence should be addressed. 1.N. Gallaher, F. P. Moody, T. R. Burkholder, and J. J. Leary' James Madison University Harrisonburg. VA 22807
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Journal of Chemical Education
