A Carbonate Project Introducing Students to the Chemistry Lab Emily Dudek
Brandeis University, Waltham, MA 02254 Student interest in a General Chemistry lab course is enhanced by the challenge of discovering an unknown and by a project approach with a research quality. In the past I have successfully engaged students in project labs for General Chemistry, such as cobalt-ammonia chloride complexes,' aspirin,2 glycine,3 and nickel-ammonia-halide complexes. These projects are not wellsuitedto the student beginning to explore a chemistry lab, because the syntheses are not simple, tlie methods of analysesrequire instrumentation, which can be hard to finance on a small budget, and the theoretical arguments involved extend beyond the first weeks of a General Chemistry course usually devoted to stoichiometry. The carbonate project, on the other hand, involves no syntheses, involves no instrumentation, and is concerned with only stoichiometry and the ideal gas law. The attractive feature of the carbonate project is it is appropriate for the truly beginning student. .. . A ~tudentis challenged to identify the metal cation in a white, water-soluhlecarbonate. Foureomplimentary methods are to be used: (1)a siniple weight loss, (2)gravimetric analysis, (3) eudiometry, and (4) titration. The white, water-soluble requirement limits the identity of the metal cation to the alkali metals. Because of cost, only lithium, sodium and potassium are actually issued to the students, but they are not apprised of this limitation. The students know only that M is an alkali metal. The project is scheduled for the first five four-hour lab periods of the Brandeis general chemistry course. Each procedure usually requires three hours rather than the four hours of the lab session, In the first period, the students merely start the gravimktric analysis as well as check in, learn about lab regulations, and clean some glassware. The first analysis based upon a measured weight loss involves weighing out a metal carbonate sample into a we~ghedflask, weighmg a graduated cylinder contaming 20 ml. nf6M HCI, pouring the acid slowly onto the solid metal carbonate,
reweighing the emptied cylinder, and weighing the flask with the fmal solution. The weight of the 6 M HCI equals the weight loss of the cylinder. The weight ofthe flask with solid MzC03 plus the weight of the 6 M HCI minus the weight of the flask with the final solution equals the weight of the COz gas evolved. From the weight of the COz gas, the moles of COz gas produced is computed, but this equals the moles of hfzC03 in the original sample. The weight of the sample divided by the number of moles gives the formula weight of the metal carbonate from which the atomic weight of M can be calculated and the identity of the alkali metal established. The experiment serves to introduce students to the use of a top-loading balance. The size of the MzC03sample is 2.00 g measured to f 0.01 g. The procedure, whichinvolves several weighings, would be too time consuming if analytical balances were used. Also the imprecision of the proce'Dudek E.P. J. Chem Educ. 1975,52,43. 2Dudek,E.P. J. Chem Educ. 1977,54,329. 3Dudek, E.P. J. Chem. Educ. 1987, 64,899.
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Journal of Chemical Education
dure would result in large relative errors if samples on the order of tenths of a gram were used. For the 2.00-g sample, the percent deviation in the formula weight determination lies between 1% and 10% The procedure is sufficiently short and simple that students have time and patience to repeat the measurements more than the requested three times in order to reduce the percent deviation below 10%. The deviation has educational significance. Students quickly learn that even though a procedure is repeated with care, the results are oRen not entirely reproducible.The 10%deviation does not preclude students from identifying the alkali metal since the five possible values of atomic weight differsignificantly, Li (6.9 gimol), Na (23 gimol), K (39 gimol), Rb (85 glmol), and Cs (133 gimol). The error in the accuracy of the results also has pedagogical value. For example, if a student determines that the atomic weight of the alkali metal in the issued carbonate is 30 gimol, is M sodium or potassium? The student is asked what errors would lead to too large an atomic weight value, and what errors would lead to too small a value, and which type of ermr is more likely? In the three subsequent lab periods, the student can check on the error argument. The question that needs to be answered is what is contributing to the apparent inaccuracy of the first analysis. Is the procedure at fault or is the sample issued impure? If the second method of analysis supports the findings of the first method, then it may well be that the problem lies with contamination of the unknown metal carbonate itself. The first analysis based upon weight loss generally leads to too large an atomic weight value by 5 15%with the percent deviation usually being less than the percent error in accuracy for most students. The major error is too small a weight loss due presumably to insufficient swirling of the flask containing the reaction mixture. The 20 mL of 6 M HCl used may also retain some of the COz gas, but the solubility of COz in this quantity of 6 M HCI should account for no more than 5% difference between moles C02 produced andmoles Conevolved. If the 6 M HCI is added too quickly, some spattering can occur causing too large a weight loss of the reaction mixture. To reduce the loss from spattering, a 250-mL flask is used rather than a beaker. The second method is a gravimetric analysis in which excess aqueous BaCIz is added to a dissolved sample of the alkali metal carbonate. From the weight of BaC03 isolated by filtration, the number of moles of carbonate can be determined, and then from the weight of the alkali metal carbonate sample, the formula weight of the MzC03can be computed. Again, in order to hold the relative error to less than 5% and to use top-loading balances that measure weights to +0.01 g, a 1.00-g sample of alkali metal carbonate is used.The sample size is not larger than this due to the limits in solubiliy of the alkali metal carbonates. In particular, LizC03has a solubility of about 1gin 100 mL of water, a solubility that decreases as the temperature of the water is increased. (Later in the year, students might be asked to explain why the solubility of Li2C03in water is smaller than that of the other alkali metal halides and why
its solubility in water decreases a s the temperature is raised whereas the solubilities of NazCO3 and KzC03 increase as the temperature is raised.) The students are directed not to heat the carbonate-water mixture to dissolve the carbonate but rather to use up to 300 mL of water in a 600-mL or 1000-mL beaker to dissolve the l g of alkali metal carbonate. Once the sample of carbonate is all dissolved, 80 mLof 0.20 M BaClz is added (students are asked to show by calculation that this volume insures a n excess of Ba2+(aa)even in the case of one a a m of lithium carbonate), and-the resulting mixture i;covered with a watch glass and heated to boiling to digest the BaC03. The mixture is then covered with a piece of plastic wrap and set aside for the next lab period. Over the interval of a week most of the barium carbonate precipitate settles to the bottom of the beaker. The weighing out of the MzC03 samole. the dissolvine of it in water. the addition of BaciZ(iq),and the heaGng to boiling is ah done in the first lab ~ e r i o doeriod of the course. Two samoles are oreoared to ;heck dn the precision of the graviAetric p;ocLdure. There is no time in the period for preparing a third sample. I n the second lab period, the analysis by weight loss is carriedout, but a t that time, the size of the BaC03particles is also checked out. If the particles appear to be too fine for easy filtering, the mixture is again digested by boiling and allowed to sit for vet another week. In the third oeriod. the solid BaCO,, is isolutcd by vacuum filtration. Two pieces of filwr oaocr arc used in the Buchncr funnel to decrease the loss df any solid in the filtrate. Often the mother liquor needs to be recycled because on the first filtration the solution remains cloudy with fine particles of BaC03 suspended in it. The precipitate on the funnel is washed with water and acetone and dried in a 110 "C oven for a t least 30 min. The precision of the formula weight determination for M&03 achieved by this gravimetric method is expected to be less than 5% and is usually between 1% and 3%. The error in accuracy is usually positive andless than 10%. The too large values for the formula weight are the result of too small weights of BaC03. The loss of BaC03 occurs in the filtration, loss in the filtrate and loss in the transfer with some precipitate remaining in the reaction beaker. This loss is somewhat compensated for by incomplete drying of the BaC03 solid. While the BaCO? is drying. the students prepare two pieces ofglass tubingto b e m e d i n the eudiome&i~analysis of the third analysis. One piece of elass tubine is simply a short piece to serve as a n lead i?om a onerhole nibtier stopper to apiece of rubber tubing. The second piece of glass tubing will be used to guide the gas, evolved from the reaction of MzC03and 6 M HC1, as it flows from a rubber tube connection to the bottom of a n inverted buret. This second piece of glass tubing requires three bends. Most students are oleased that thev can manioulate elass tubine and that w6at they produce is usefui in a&bsequeG procedure. In the third analysis, the CO2 gas evolved from the reaction between M2CO3(s)and HCl(aq) is collected over a column of water and the number of moles of COz gas is determined from pressure, volume, and temperature measurments. The 50-mL buret, limits the size of the M&03 sample to about 1.3 x lo3 mol. Now the solid sample is weighed out on an analytical balance to the nearest milligram. In order that the solid and HCKaq) do not react until the system is closed, the sample is transferred to a small vial (about 5 mL volume). usuallv bv slidine the weighed sample down the crease i f a pieceof powder paper into the vial. Both ends of a string (about 6 in.) are tied to the mouth of the vial in such a way that the string serves a s a bowed handle.The vial with solid is placed upright in a 125-mL Erlenmeyer flask which contains 10 m i of 6 M
HC1. The string is pushed down into the flask and then the flask is rappedby
