The Density and Apparent Molecular Weight of Air A Simple Introductory Experiment Arlo D. Harris California State College, San Bernardino, San Bernardino, CA 92407 Fundamental to a study of chemistry is an understanding of both extensive and intensive properties. Most inroductory courses utilize mass and volume as examples of the former. T h e ratio of mass to volume equalling density is most often used as an example of the latter. These concepts are quite easily defined and described in lecture materials, hence they require little effort on the part of the student for understanding. Generallv sweakine. the first exoeriment done in most introductorylal&ato$ courses deils with physical measurements. The aforementioned concepts and definitions lend themselves readily to simple measurements to confirm them as basic tenets of chemical science. A variety of measurements of mass and volume are used to demonstrate the density of solids and liquids, both pure and solutions. These can be found published in many commercially available laboratory manuals. Some institutions use modular units or use hand-out experiments of their own. Few, if any, programs incorporate a measurement of the density of a gas a t this early stage in a laboratory sequence. Possibly there is the idea of a need to include a discussion of gas laws which, of course, comes later in most courses. In fact, many experimental programs use the classical Dumas method later in the course to determine the molecular weight of a volatile liquid or even a gas itself. The exoeriment described herein can be used bv beeinnine students without the mention of gas laws. It re& sGely on mass, volume. and temoerature. The variables are mass and temperature while the v h n e of the system remains constant. Although this experiment uses the ideal gas law in deriving the working equation, students need not be burdened with it. It is sufficient to instruct the student in simple concepts which are presented a t the end of the derivation. Apparatus (as shown in the figure) 250-ml Erlenmeyer flask 1-hole rubber stopper 1 short piece of glass tubing (about 10 cm long) I short piece of rubber tubing (about 8 om long) 1 pinch clamp 1 800-1000-ml beaker Experimental Procedure The glass tubing must go through the rubber stopper so that it is
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Journal of Chemical Education
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RUBBER TUBING AND PINCH CLAMP
ONE HOLE RUBBER STOPPER 250 ml ERLENMEYER FLASK
APPARATUS FOR DETERMINING THE DENSITY AND APPARENT MOLECULAR WEIGHT OF AIR Apparatus used to determine the denslty and apparent molecular wetght of air
flush with the bottom and will not extend into the flask. The rubher tubine slinsover the extendine end. The stouner is oushed verv tiehtlv
to four decimal places at mom temperature.The temperature must be measured to within O.l°C. The apparatus is then clamped in such a way that as much ofthe Erlenmeyer flask is inside the large beaker as is possible (see figure). The beaker is filled with water to the verv ton allowine onlv enwrh room for huhhling due to rapid boiling. T i e water is br&htta a
