A Student Research Style Report Project for the General Chemistry Laboratory Frances Dunkle Coffin Baldwin Wallace College, Berea OH 44017 The preparation of a report involving the results of several experiments performed in the general chemistry laboratory offers a n excellent learning opportunity for the student. The student learns to evaluate data and to combine the results of several experiments in order to identify two unknown substances, a metal and a n organic liquid. The identifications are made from the experimental measurements of the density, the molecular weight, and refractive index of the liquid and from the density and specific heat of the metal. The atomic weight of the metal is estimated from Dulong and Petit's rule. Experimental Three l a b o r a t o ~sessions are reauired to determine the properties of the &knowns. ~ x ~ e r i i n e n tmethods al for the measurement of density, the specific heat of a metal, and the molar mass of a volatile liquid are generally available in most laboratory manuals for general chemistry. The text ts usedin our laboratory was Slowinski (I),~ x ~ e r i m e n1,7, and 11. The operation of the refractometer is described during the prelab discussion period, and the students are supervised while making the measurements. Evaluation of Student Data The laboratory reports are graded, and the student is given a n error range for the data; i.e. c+ 2%, fl-5 %; >L List of Unknowns for Student Reports Metals Atomic Number 13 56 83 20 24 29 26 82 12 80 42 28 14 38 50
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Handbook Number (4 Name aluminum 175 barium 204 bismuth 262 calcium 3918 chromium 4717 copper 5491 iron 6708 lead 6881 7522 magnesium mercury 7760 molybdenum 9092 nickel 9098 silicon 10027 strontium 10257 tin 10455 11963 11971 14060
Liquids Name acetic acid ethyl acetatea l-propyl acetate 2-butanone carbon disulfide(H) cyclohexanea ethanola trichloroethylene (H) heptane n-hexanea carbon tetrachloride (H) chloroform (H) n-octane paraldehyde l-penianol n-propanol-2-methyl 2-propanola toluene
5%.Ranges are used to give the student an idea of the reliability of the data. (Error ranges are generally higher in the determination of the molar mass of the liquid.) Students with large errors in their data are expected to repeat the experiment to improve the accuracy of their measurements. Analysis of Data Students are eiven a n instruction sheet (see Annendix). ~ ~ specifying the type of report expected, and tables of'15-20 Dossihle metal and llau~dunknowns. !See the table for lists bf elements and compounds included.) The student is exa t e in the Handbook ~ e c t e dto look UD the a ~ ~ r o ~ r ivalues bf ~hernistrya n c ? ~ h ~ s (2j. & The Handbwk number of the organic liquid is fiven to re vent confusion with the unfamciar terknolo* of organic compounds. By comparing the experimental values with the tables, it should be possible to identify the two unknowns.
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Comments Five metals and five liquids are employed as unknowns, but not all of the elements and liquids shown in the table can be used. I n many cases, the known properties of the substance will preclude its use (e.g., mercury or acetic acid). Also, the boiling points of the liquid unknowns must be well below 100 -C. However, elements and liquids should be chosen to give a range of properties from which the student selects the proper element or compound. I t is advisable to provide extra laboratory time for students who wish to repeat a n experiment. At the end of each laboratory period, the student enters his or her raw data into a computer spread sheet. This permits the instructor to evaluate the experimental results without waiting for the laboratory report, and i t also eliminates the possibility that the student's computational errors will lead to a false sense of security concerning the data. Student Reports Student reports followed the assigned format and included tables comparing their experimental values with the handbook values for each property of the liquid or metal, to support their identification. Part of the process in enabling them to prepare a "good" report lies in providing them with the detailed outline of the contents ofthe report. Conclusion The preparation of a 'research-style" report in a general chemistry laboratory section provides the students with a n opportunity to learn to evaluate experimental data and leads to a better understanding of the necessity for reliable measurements. I t also offers a n introduction to the preparation of a technical report. The students show unusual interest in their experimental work and generally turn out satisfactory papers. Literature Cited 1. Slowinrki. E. J.; wolsoy, W. c.;Masterton, WL. Chemical Principles in :he Lobom-
'suggested
liquids for use
(H)= hazardous -do not use
852
Journal of Chemical Education
t o n . 5th ed.: Saundera: NeuVork. 1989 -~~~ ~
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2. ~ e a s ; , ' C., ~ . ~ d CRC . Holndbook ofChemisfni and Physics, 61st ed.: CRC: Bms Raton, FL. 1980.
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Appendix: Laboratory Report for General Chemistry Identification of a n Unknown Solid and Liouid Near the beginning of the quarter, you will be given two numbered unknowns. a metal and a liauid. You are exaected to identifv . them from the re& of ~ x p ~ r i m e k1,s7. and 11; plus a determi-' nation of the refractive index of the liquid. ~~
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The form of your report should be similar to that used in chemical journals found in the departmental library. Typical journals such as The Journal of Chemical Educationor the Journal of the American Chemical Societywiil show the correct format.
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From these experiments, you will be able to determine the following properties of the unknowns: Metal Density Specific Heat Atomic Weight
Liquid Density Molecular Weight Refractive Index
Your ability to identity these unknowns will depend on the accuracy of your experiments. When the laboratory report for each experiment is graded, you will be given an estimate of the error in your determination. The following error ranges will be used: Acceptable Dubious Unacceptable
&2% error 2-5% ermr >5% error
The report must be typed. It should have the following sections: Title: Abstract: Method: Data:
Calculations: Discussion:
The repoll must have a title. Summarize your results very briefly. A brief reference to your laboratory manual is sufficient. Tao~ ate tne va Jes oota ned for each property of the Lnmowns Be s ~ r e to nc m e me dent fy ng numbers for tne Lnmoww, ana g ve the r error ranges. Show a typical set of calculations for each property. Make a tentative identification of your unknown from the values which you have determined experimentally. Compare these with the values for the compound found in the Handbook of Chemistrvand Phvsics. Use a table to make these cor;lparisoni, and compute the percent difference for each.
Volume 70
Number 10 October 1993
853
