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JAMES 0.SCHRECK Univenity of Northern Colorado Greeley,CO 80639
filtrates and residues Synthesis of an Insoluble Salt A Stoichiometry Experiment Erling Anthony Arrowhead High School, Hartland, WI 53029
I have found it advantageous to conclude the stoichiometry unit in my high school chemistry course with an independent, problem-solving experiment. Each student (or student group) is assigned the task of preparing 2.0 g of a different water-insoluble salt (for example, sulfate or carbonate of barium or calcium). Students are given a list of available stock chemicals. They are required to obtain their cation from an instructorrprepared stock solution (40.0 g of suitable salt per liter of solution) to minimize inhaling airborne barium and strontium dust. This requires a calculation based on solution concentration and can be reviewed again when molarity calculations are covered later in the course. After the students choose a reactant salt and complete the necessary calculations, I evaluate the experimentwith the student. I emohasize safetv rmidelines and precautions (including handling barium ahlstrontium saks, clean-up procedures, etc.), verify their calculations making sure they have included the appropriate water of hydration in their calculations, and review their experimental procedure. With respect to the experimental procedure, the students are reminded to wash the precipitate if they have not indicated this step, as well as, to test for excess starting reagents. The students are then allowed to proceed with their laboratory procedure. Once the procedure is completed, their results and the product (for proper disposal1) are submitted to me. A sample student report including procedure, data, and calculations is given below. Most students achieve reasonable results (about 90% isolate 2.0iO.l d i f t h e appropriatesalt ischosen. Students not achieving reasonable results find calculation errors and indicate these in their laboratow reports. I typically assign the least soluble of the sulfite; suifate, c&bnnat& phate, and oxalate salts ofharium, strontium, and calcium. Upon wmpletion of the experiment, I wmpile data including starting materials fmm each student group and post the results for comparison. Frequently interesting patterns emerge. For example, three independent groups produced barium sulfate obtaining 1.96 g, 1.92 g, and 1.98 g using sodium sulfate, copper(I1) sulfate pentahydrate, andpotassium aluminum sulfate dodecahydrate, respectively. Two other groups produced strontium sulfate from sodium sulfate obtaining 2.01 g and 1.93 g. Students find these vari~
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Inc , P 0 Box 21 9. 'Tne F Inn Sc enld c Cata og (Fl nn Sc~entll~c, 131 FI nn St Batav a. IL 60510.0219, o~tlmesa d~sposalproceoLre for barium compounds. However, I prefer to store the products in labelled bonles since the total waste involves about 10 g of each compound per year.
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Journal of Chemical Education
ations interesting and, as a result, gain a better understanding of the experiment. Student evaluations indicate they enjoy the challenge of designing their experimental procedure and solving problems encountered during the exercise. These problems include discovering an easy . way of checking for excess starting materials aLd choosing an acceptable reactant. Some students have attempted to produce barium sulfate by reacting barium chloride with calcium sulfate. After carefully weighing the latter salt, they find it to be insoluble in water and realize the importance of consulting a soluhilitv table. Other students are further challenged when I assign a substance that orecioitates as a hvdrate. After carefully drying the substAce i t low temperatures, they fmd the; have over 2.00 e of oroduct. After verifvina their calculations and resulg, I ;uggest they consuk the Handbook of Chemistrv a n d Phvsics for information about a hydrate of their prepared s a t . This leads to the idea of drying a t elevated temperatures and to acceptable results. Student comments indicate that this is a valuable exercise. They express an increased understanding and appreciation for stoichiometry and greater appreciation for the detail that is necessary in conducting a successful experiment. ~
Typical Student Laboratory Report Purpose
To prepare 2.0 g of barium phosphate Procedure 1. Two reactant salts were chosen. A stock solution supplying the barium cation was obtained. Sodiumphosphatedodecahydrate was chosen as the source of the phosphate anion. This salt was
chosen because the other two phosphates on the chemical inventory list (calcium phosphate and lead phosphate) are insoluble in
water. 2. The required stoichiometric calculations were completed (see below). 3. Weighed 2.52 g of sodium phosphate dodecahydrate and dissolved it in 25-30 mL of water in a 150-mLbeaker. 4. In a separate 150-mLbeaker, obtained 51.75 mL of barium chloride solution fromaburet and addedit to the beaker containing the sodium phosphate solution. 5. Stirred the mixture to ensure reaction. 6. Weighed a sheet of filter paper, folded it, and placed it in a short stem funnel. The mixture from step #5 was filtered on the funnel. 7. The filtrate was tested for unreacted starting materials by adding a few milliliters of the filtrate to each of two test tubes. To one test tube was added a few drops of barium chloride solution.
h'ochangcorcurrcd. To the other test t u h ~ was added a few drops ofsod~umphosphatesolution. .\ hint t r u c c o f u h ~ solid~ppearrd. e 8. The aohdun ihr filter onoer was washed with 20 ml. distilled water and allowed ta dry &;night. 9. The solid was weighed and submitted to the instructor.
4. Volume of stock solution of barium chloride needed to produce 2.00 g of Ba3(P0&
Calculations
r mL of BaClz solution =
3BaC12+ 2Na3P04
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Ba3(P04)2+ 6NaC1
1.Moles of Ba3(P04)z needed to produce 2.0 g of Ba3(P04)z
5. Percentage yield = 3.32
n lo3 mol Ba,PO&
1 97x 100 = 98.5% 2.00 g
2. Grams of BaClz needed to produce 2.0 g of BadPo&
3. Grams of Na3(P04)z.12Hz0 needed to produce 2.0 g of Ba3(PO4)2 x g of Na,(PO4),.l2H,O =
Data 1. Mass of weighing paper
1.07g
2. Mass of weighing paper + Na(P04)z .12Hz0
3.60 g
3. Volume of BaCI2 solution needed
51.75 mL
4. Mass of filter paper
0.76 g
5. Mass of dried filter paper + dried Ba3(P04)2
2.73 g
6. Mass of dried Bas(PO+
1.97g
Volume 68 Number 12 December 1991
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