A Laboratory Skills Test Determining Students' Ability To Transfer Specific Volumes of Liquid Richard S. Mitchell Arkansas State University, Jonesboro, AR 72467
Laboratorv exercises for second-semester general chemistry frequently require the student to elrect the qunntitative transfer of liqulds. Grades are assessed on the basin of resultsobrained by thestudent'scalculations. Therecorded volumes simificantlv a t k t the results. It is impossible to observe eaih student in a laboratory section every time routine operations must be used. One would suspect that after using a pipet several times in three experiments the student would develop some skill with a pipet and bulb. Frequently, this skill is not achieved. The author wished to identify those students who had failed to achieve an acceptable level of expertise. Such an exercise should exclude any reasons why one student obtains better results than another, except for the difference in the skill of measuring and transferring volumes of liquids. The student would then recognize hisher need of additional coaching in this area and presumably will be receptive to this additional instmction. Objective Ted of Lab Skills Pickerine and Kolks' evaluated the abilitv of teaching asslstants';~grade lab technique. They concl"uded that n i obicct~vctest of lab skills is iustified. I'ickerind has discuked some of the reasons wky students fail at volumetric analvsis. MacNevin3 reported on a Faian analvsis as a lab practical in quantitative analysis. "Students can see for themselves how reliable thev are bv com~arisonwith their associates They are impressed gcm?raliv by the high demeeofexcell~nceofalarrre numherofthcclass Convcrselv, those who are less capable or who have been careless inefficient have it brought home to them in a forceful way by seeing their results among the rest." The lab practical developed by Jones' impressed the author by its sheer simplicity. The practical measures only the ability of the student to weigh a sample, quantitatively transfer that sample to a volumetric flask, and dilute the resulting solution to the mark. Except for the possibility of a contaminated salt or an improperly calibrated flask, any deviations from the class average are attributable directly to the work of the individual. General chemistry students are expected to be skilled in the use of volumet& equipment bythe end of their first semester. They have observed the demonstration of the equipment and used the equipment in three or more experiments. A casual observation supports the hypothesis that many students have yet to develop competency with volumetric measurements.
' Pickering, M; Kolks, G. J. Chem. Educ. 1976, 53, 313. Pickerinq, M. J. CON.Sci. Teach. 1979, 8, 169. ~ a c ~ e iW, n J, Chem. Educ. 1961,38,144 4Jones,M. M. J. Chem. Educ. 1977,54,178.
The Experiment The students are furnished with a standardized base of approximately 0.1000 M. In addition, they are to obtain an aliquot of an approximately 1.000-M unknown acid in a clean, dry beaker. Dispensing of these reagents was monitored only to insure that sufficient quantities were available for other laboratory sections. The students can see that all solutions come from the same common source. The students were instructed to clean all volumetric glassware and were reminded to rinse the glassware with the solution to be measured. Each student was to transfer a 10-mLaliquot ofthe acid by pipet to a 100-mLvolumetric flask and dilute to the mark with distilled water. Atter mixing, 25-mL aliquots of the diluted acid were to be transferred to each of three Erlenmeyer flasks, phenolphthalein added, and the acid titrated to the persistent pink endpoint. The students were informed that their individual titration volumes would theoretically be identical if the solution was homogeneous. They were urged to repeat the exercise if any of thc thrcc values differed significantly. Also, they were encouraged to compare their averaged result with their colleagues. When q;eried, the instmetor stated that they should have a range no greater than 0.30 mL if they had exercised due caution in their work. The immediate response of the students was most gratifying. Many students were observed practicing with the pipet and bulb before starting the experiment. The Results Fiftyfive students in two sections performed this experiment. All calculations were checked for accuracy. The mean ofthe reported averaged volumes was 26.80 mL with a standard deviation of 1.23 mL. Eleven reoorted averaged values were arbitrarily discarded for gross deviation from the mean. The mean of the 44 remainine values was 27.07 with a standard deviation of 0.28 m~.-?hemeans of the individual sections were 27.05 and 27.04 mL. Only one set of equipment generated excessively low values. indicating the ~ossibilitvthat it was incorrectlv calibr&d. The rest of ihe rejedkd reports indicated that the students had ignored the suggestion to compare their results with others in the lab section. The most blatant deviation was from the person that obtained an acceptable range of titration volumes while using almost 6 mL less than the mean value. The students pot the message that poor results could be attributed to technique. ~;rther, pipets was obvious during student competent, in the use of . . the rest of the semester. Chemical Education Resources ofpalmpa, PA, is preparing a "lab separates" procedure for this experiment.
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Volume 68 Number 11 November 1991
941
