Introductory Laboratory Exercises Susan Wells Rollinson Dabney S. Lancaster Community College, Clifton Forge. VA 24422
A perennial problem with General Chemistry courses is how to handle labs during the drop-add period, when class rolls are fluctuating. I wish to describe the exercises we use for the first two weeks of General Chemistry that minimize the problem. As these procedures do not require the use of individual equipment, student lab drawers are not assigned until the third week, when enrollment has stabilized. High, school chemistry teachers might also want t o consider these exercises since they require minimal equipment and are easilv a d a ~ t e dto suit the instructor. -~his'introductorylab consists of several procedures. Some are performed as demonstrations. Others are performed by groups of two students. In two cases, the exercise requires data contributed bv evew class member. The nature of the procedures also make them serve as an "ice-breaker", introducing the students (and instructor) to one another. I usually have a computer in the lab on which the datacan be "instantly" reduced and plotted. At the conclusion of the experiments, eachstudent must present his or her data analysis in a brief lab report. Learnlng Oblectlves These exercises can be used to fulfill several educational objectives commonly found a t the beginning of general chemistry. The student will be able to use the triole beam and analvtical balances. use metric'system units of mass, length, and temperature, graph linear relationships and reduce simple nonlinear relationships to linear ones, determine the slope and intercept of a linear graph and extrapolate to extreme or impossible experimental conditions, use significant digits properly, understand the relationship between significant digits and the precision of laboratory measurements,and use computer spreadsheet programs to reduce data and perform linear regression. Additionally, these exercises provide material for a review of algehra and help students acquire confidence with the metric system and the chemistry lahoratory. Llnear Graphs We start with a selection of simple experiments that produce linear graphs. Using either a physician's scale or a bathroom scale. the mass of a student is measured in kilograms while hoiding 2,3,5,7, and 10 copies of the chemistry textbook. (Choose a volunteer with strona arms, as 10 textbooks may weigh 50 pounds!) Plotting t h i mass against the number of textbooks produces a linear graph. The mass of the textbook can be found from the slope of the line, while the intercept reveals the mass of the volunteer. The next experiment gives an estimate for the value of absolute zero. A hollow metal sphere filled with air is connected to a Dressure -eauee " .(Fie. - I).' . A minimum of four pressure readings are taken a t temperatures ranging from 0 to 100 OC. The resultine linear e r a- ~ of h temnerature aeainst pressure is extrapolated to zero pressure and an estimate for absolute zero is made. The last "linear" experiment requires taking the temperatureof several water baths with both Fahrenheit and Celsius thermometers. (Icelsalt, ice, cold water, room-temperature water, hot water, and boiling water make a convenient range
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Figure 1. Fmssure-volume apparatus.
Wooden Handle
Hollow Metal Sphere in Temperature Bath Figure 2. Absolute zero apparatus.
of temperatures.) A graphical comparison of the temperature scales is made. and the exoerimental eauation is compared to the exact relationship between the temperature scales. Nonlinear Graphs We do two experiments that result in nonlinear graphs. First we do a P-V plot with data taken from a 30-mL syringe connected to a pressure gauge (Fig. 2).= The students are led to discover that a plot of P against 1/V (or V against 1/P) is linear. Second, the massof a seriesof rubber stoppers (six toeight stomers raneine in size from 00 to 8) is comnared to the avlrige of tce tbp and bottom diameters.3 0; course, the stonners must all he made of the same rubber! A nlot of mass ag&t diameter gives a parabolic curve and the students are led to discover that mass vs. the square of the diameter is a linear relationship. I
Absolute Zero Unit. Educational Materials and Equipment Compa-
ny. 46 Lafayene Ave., New Rochelle, NY 10801. Available through
Saraent-Welch (cat. no. 16021or Frev Scientifictcat. no. 75361. ~ o ~ l e ' s L a w ~~duca1io"ai k1. ~at&iaisand ~duipmentcompany. Avaiiaole through Sargent-Welch (cat. no. 1077D) or Frey Scientific (cat. no. 7536). Abraham, M. R.; Pavelich, M. J. Inquiries into Chemishy; Waveland: Prospect Heights, IL 1979; P 16. This text is a good source for other "guided inqurry" experimerk ~
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Volume 65 Number 2
February 1988
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"Scatter" Graph
Each student must contribute hisher height in centimeters and mass in kilograms to a class chart. The instructor and lab assistant also participate. A plot of height against weight usually gives widely scattered points around a trend toward taller people weighing more than shorter people. All height and weight data are, of course, anonymous. This, along with the metric units, has prevented complaints of embarassment! We also discuss whether other variables, such as age, should he considered. Precision of Measurement
A final exercise is performed to give the class afeel for the precision of laboratory balances. Four objects (a sheet of filter paper, a hall-point pen, a 100-mL beaker, and a rubber stopper) are weighed on both the triple-beam and analytical
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Journal of Chemical Education
balances. Then all four objects are placed on the pan of each balance and weighed. Everyone in the classuses the identical objects and balances. The class data are pooled and several comparisons are made: The calculated sum of the four objects is compared with the measured sum. The average mass of each ohject and the average deviation for each is calculated. The precision of the triple-beam and analytical balances are compared. Summary
The use of several short introductory experiments has allowed us to present meaningful experiences to our general chemistry students without having to assign lab drawers in the first two weeks of the term. These exercises introduce the students to the metric system, balances, significant digits, graphing, and computerized data analysis.
