Measurement of Density A First Laboratory Experiment for Beginning Chemistry Students W. S. Richardson and J. E. Teggins Department of Physical Sciences, Auburn University at Montgomery, Montgomery, AL 36193
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The instructor for the first laboratorv meetine in a colleee chemistry course faces two common problems a t many colleges and universities. Firstlv, there has been insufficient time available to cover a meaningful amount of theoretical chemistrv in the lecture portion of the course. In addition, many otherwise competent students (with average cummulative ACT scores of 22 a t our institution) have acquired little knowledge of chemistry in their precollege years. Previous contributions to this Journal have suggested excellent experiments for the honors chemistry student.' The majority of college chemistry students a t many institutions, however. could not be exnected to ~ e r f o r ma t such a level a t the start of their studies.;l'he challenge, therefore, is to provide a first experiment that Drovides insights into scientific Drocedures without presupposing an; detailed knowledge of chemistry on the part of the student. By the time of the first laboratory experiment, it is common to have reviewed metric units and basic mathematical procedures. In a small fraction of a laboratory period it is possible to cover the additional background in graphical procedures, significant figures, and the definition of density required for students to understand the described density determination experiments. Simple equipment is employed for these procedures. The experiments are unusually inexof renensive to ~ e r f o r mbecause the modest ouantities Hgents required may be used repeatedly. The experiments that meet our needs for the initial lahoratory period require the student to measure the densities of liquids and solids bv measuring successive masses and volumes of each subs&nce and, using the data to prepare graphs, to calculate the density of each substance from the slope of the appropriate graph. The collection of data provides an introductory experience with basic laboratory equipment while the graphical determination of the density, as opposed to the calculation of the density from the average of several measurements, makes a vivid impression of the potential error in the measurements. The student also experiences one of the most common methods of data treatment in science, the use of the graph to see trends in data. Experlmental The experiment requires the use of a 100-mL graduatedcylinder, a triple-beam balance that weighs to f0.1 g, two 200-mL beakers, weighing paper, several liquids such as water, isopropyl alcohol, or ethylene glycol, and solids such as zinc, tin, or aluminum. Density of the Liquids Weigh a clean, dry 100-mL graduated cylinder on a triple-beam balance to *0.1 g. From the stock bottle obtain in an beaker 80-100 mL of the liquid to be measured. Pour approximately 10 mL of the liquid into the graduated cylinder and measure the volume to *0.1 mL. Weigh the cylinder with liquid to *0.1 g. Add approximately 10 mL more of the liquid to the cylinder, record the total volume, and
' For example, Olmsted, J., Ill. J. Chem. Educ. 1986, 63,538.
* Weast, R. C., Ed.
Handbook of Chemistry and Physics; CRC:
Cleveland. OH, 1974: p C-453.
weight. Repeat the process until rra portam of liquid have heen aodrd tu the cylinder. Return the hquid to the stork hortle, clean and dry the cylinder, and repeat the pnredure for a sw
