A Simple, Safe, and Inexpensive Laboratory Exercise in the~uidedInquiry Format John M. De M w r a University of Idaho, Moscow, ID 83843 Joseph A. Marcello Sheldon Jackson College, Sitka, AL 99835 There are several introductory lahoratory exercises in chemistry t h a t illustrate stoichiometry, limiting reagents, and proportionality. We chose t h e following reaction for the same purpose: CaC12(.,,
+ 2NaOH(.,,
-
C&kDz
+ 2Na'Cl-
Both reactants are cheav and readily available. Sodium hydroxide is used routinely in student laboratories, and users are well aware of its corrosive property; thus, only the usual cautions are necessary. T h e &oduc& calcium hydroxide and sodium chloride, are relatively harmless. Spills are easily removed from glassware, apparatus, lab benches, sinks, drying ovens, and balances. These are obvious advantages over such products as lead chromate or lead iodidel.2, which pose disposal problems. We d o not reouire meticulous techniaue for filtering, washing, and drying of precipitates. By making measurements t o three sienificant figures, students obtain illustrative quantitative data. This exercise is easilv completed in a three-hour lab periand there is also enough time od by students woking in for in-lab discussion between lab partners and among student pairs. We require rhat the work be completed and a revort handed in 1)rfore rhe lab is over. Only ordinary lab glassware and equipment are required. Electronic balances and a drying oven are the main items. The Procedure (to the Student) Part A Two solutions are available on the side bench. These are CaCI?. 2H20 1.00 M and NaOH 2.50 M. Obtain about 30 mL of each in two clean beakers, and fill a 25-mL buret with each solution. Run 5.00 mL of the calcium chloride solution into each of five large test tubes (20 X 150 mm are suitable) numbered 1through 5. Then, add 2.00, 3.00, 4.00, 5.00, and 6.00 mL of NaOH solution to test tubes 1-5, respectively. Mix the contents of each tube well. Save the remaining solutions in your burets far use in Part C. Number and weigh fivesheets of filter paper (Whatman #50,11cm paper or Fisher P5 are two good choices for speed). Filter each of the five reaction mixtures. Catch the filtrates in separate clean, labeled test tubes or beakers. Wash each filter twice with 5-mL volumes of water, and save the filtrates for use in Part B. Carefully remove the filter papers with precipitates from each funnel. There is no need to unfold them. Put them on a paper towel in an oven to dry (about 110 "C, 40 min). Remove the paper towel and filters from the oven, and allow them to cool on the lab bench for about 5 min. Weigh the filters. Pari B While the filters are drying, do the following tests. Use large test tubes. 'Abraham, M. R.; Pavelich, M. J. Inquiries into Chemistry; Waveland: Prospect Heights. IL. 1979. Slowinski, E. J.; Wolsey. W.: Masterton, W. L. Chemical Princb ~ l e in s the Laboratory; Saunders: Philadelphia, 1983.
452
Journal of Chemical Education
I . Dlaadve about I g ofsolid CaCll dihgdrate in about 20mLdcionized water. Inn wcond tube diwolw 1 g NaOH in 20 mL water. Pour haliof
enrh n h t i o n into mother tubr, and m u well.
2. Assume that ihr reaction p u observ~dwhen you mixd the two snlutitms is the rrsult of combination of rms in solution. Ucr chemicalsymbols turhgwwhat those ionsare and hon they might
combine. 3. Predict the identity of the white precipitate. Test your prediction
using those chemicals on the side bench (KOH, NaOH, NaCI, KC1, CaC12, Ca(OH)2,Ca(NO&, CaSOA. Your lab instructor will not help unless you have made some attempts. Discuss your approach with your lab partner or insmall groups with your instructor. 4. Tabulate the results of your tests, and write a balanced chemical equation for the reaction of calcium ehoride with sodium hydroxide. Part C Divide each of the filtrates that you saved from Part A into halves. Use test tubes of the same size. What is the limiting ion in each filtrate? Add 10 drops of calcium chloride solution from your buret to one half of each filtrate and likewise 10 drops of sodium hydroxide solution to the other half of the same filtrate. Note what happened. Use sign to indicate there was precipitation or a - sign if there was none. Use the information in your data sheet (CaC12 solution (mL), CaC12(mmol), NaOH solution (mL), NaOH (mmal), filter paper (g), filter paper and precipitate (g), precipitate (g)) to complete your mental analysis of these results, and tabulate them.
+
Calculations Calculate the number of millimoles of product recovered in Part A. Use the formula weight of the product that you identified in Part B, no. 3, above to make that calculation. Assume that the concentrations of the solutions that you used in Part A were exact and that your lab technique was flawless. How many millimoles of precipitate would you expect to get from each reaction? Briefly discuss your data and results in terms of the purposes of this exercise (see pre-lab). Dlscussion Some of the drawbacks of calcium hydroxide are t h a t it is slightly soluble in water (about 1.7 g/mL) and t h a t it readily absorbs C 0 2 from the air. T h e solubility factor causes an estimated loss of about 0.02 mmol in the predicted yield from each reaction. T h e solubility product for a saturated solution of Ca(OH)2 with very low ionic concentration is Analysis of Filtrates Filuate 1 2
3 4 5
+
Limiting ion OHOH(none) Cs2+ Cast
Excess Ian Ca2+
Ca2+ (none) OHOH-
Added Ca2+
-
+
++
Added OH-
+++ ++ -
A means a precipitate was obaervad. (me number of +o indicates the relative amount 01 precipitate.) A - means no precipitate waa observed.
about 10-5. The K,, in these reactions ranges from 4 X lo-' in tube # 1 to 9 X lo-' in tube #5. We think that these considerations are quantitatively negligible in this exercise, but such possibilities may be introduced by lab instructors to stimulate discussion. Laboratory instructors should be patient and allow students to express themselves. We find that students are at first hesitant, hoping that the instructor will provide leads and answers fudiffic~lltiesthat arise. For example, in Part B, no. 3, some form of test grid usually evolves out of the discussion. Some advance information is helpful to students, so we hand out copies of a pre-lab and the actual lab exercise one week ahead of schedule. They are asked to work through the
lab and to try to answer questions that are raised in the text. Copies of the pre-lab are available from the authors. This exercise solves some of the difficulties of introductory student laboratories. One difficulty is that many students cannot think about manipulative details and theory a t the same t i n ~ e .We ~ . ~also agree that meticulous technique, though essential for the accom~lishedchemist. is not of prim& importance at the intrdductory leve1.5 kinally, no .iustification is needed for the honuses of safer!, and h w cost. Pickering, M . J. Chem. Educ. 1985, 62, 874.
'Schrader, C. L. J. Chem. Educ. 1984, 61, 1001. Hanson, A. L. J. Chem. Educ. 1982, 59.671.
Volume 64
Number 5 May 1987
453
