Mark M. Jones' Vanderbilt University Nashville. Tennessee 37235
A Simple Practical Lab Test for Freshman Students
Many teachers of freshman chemistry laboratory use the analysis of unknowns as the method of choice for testing the proficiency of their students in laboratory technique. Such procedures are generally too difficult or too time-consuming for laboratom courses for nonmaiors. esoeciallv durine the first semeste; course. For many 01these'studeita, however, it is imoortant that we check to see that thev know how to win training with the theory they&e taught because their manv of them will use such techniques later in their careers in the health sciences. The e x ~ e r i m e ndescribed t here uses the preparation of a solution df designated molarity as a test ofstudent competence. 'I'he material wed, K2Cr04, produces solutions of an intense yellow color whose optical density follows Beer's Law. As a consequence, the accuracy of the students' work and calculationscan he checked immediately by the instructor via measurement of the optical density of the solution prepared and a comparison of this value with that required fromacalihration curve. While this is not a precise check of the students' work. i t does allow a r a ~ i madine d of the results. It allows the sorting out of those sludLnts whb do not understand how to prepare a solution of designated concentration and an assessment of the skill and care utilized by the students in carrying out their work. Procedure for the Practical Lab Test Before the class begins the test, it is necessary for the instructor to construct a calibration curve of optical density versus concentration. For the preparation of the solutions,the same equipment should be used as is to be used by the students in their work. This allows reasonable conditions to be selected for student use. For example, if analytical balances are used with a typical spectrophotometer, the practical range of molarities to be assigned is from 0.0008 M to about 0.0100 M. The upper limit is set by the extinction coefficient of K2CrO4at the wavelength used. For this purpose it is convenient not to use the wavelength of maximum absorption (366 mp), but rather to use a wavelength at which more concentrated solutions give readings within the range of the instrumentation. For the experiment as carried out here, a wavelength of 475 mp was used. It is thus measured at a wavelenath of the ab..~eorresoondine to a more "flat" oortion . rorprion curve, so the ahiurbanre readingswill be more precise than trthrrwise. A Beckman mtadel H spertmphot~,meteror a Bausch and Lumb Spertnmic 90 are adrqunrc for the measurement of optical density unless greater precision is required in grading. In such cases the calibration curve and the grading can he done with s recording instrument which will measure the optical density to f1%or so. A calibration curve used with one class is shown in Figure 1.The central line is the Beer's Law plot of optical density (at 475 mp) versus molnritv of KoCrOa. " The laboratory procedure is quite straightforward. Each student is assigned a different designated molarity and told to prepare a solution of K&O1 of that strength. The equipment required is provided and consists of an analytical balance, reagent grade KzCrO,, weighing boats or bottles, and volumetric flasks. Each student is also provided with a brief set of instructionsand adatasbeet (Fig. 2). Thestudents calculate the weight of KzCrOl needed to preparethe amount of solution correswndineto the volume of the volumetric flask furnished. Thrg then proccpd t o weigh out the amount of 1i)CrOd required nn accurately as pussihle. This moterial is then quantrtatiwiv transferred tu the v d u n i r t r ~flask, ~ rhr solution mad? up to volume, mixed
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OPTICAL DENSITY Figure 1. Calibration line and grading pattern. The cennal heavy line is a plot of the optical density of K2CrO4~olutions at 475 mp versus rnolarity. The n u m bered bands on eimer side give me grades fwstudent wrwons wim t% enigned molarities and optrcal densities with various deviations from the calibration line
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'I wish to thank Drs. B. A. Hess, Jr., and T. H. Pratt for indispensable assistance in testing this out in the laboratory. 178 / Journal of Chemical Education
Figure 2. Data sheet used by each student.
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thoroughly, and taken, with the data sheet, t o the instructor for measurement of the optical density and grading. The instructor takes the students'solutions, washes out a cuvet with them twice, then fills the cuvet and reads the optical density. The measured optical density is compared with the expected value from the calibration curve and the students' grades can be read off the graph, recorded, and given t o the students. Any problems that are encountered by the students can then be discussed on an individual haaia. ~
Using the rather lenient grading scale shown in Figure 1, a class 01 12'; freshman nursing students had a grade distrihvtion of37 a t 1W, 38 at YO, 49 at 84.and 3 at 70. While the rnlihration rurve must be
.ormared . hv the instructor hefore the laboratorv is held. the eradine " -
hands can he set after the student
