-:m
Boyle's Law with Plastic Syringes
7
N.
0 x >2
.
7
7
0
2
4 MASS, Kg
6
8
Since the publication of the article by Davenport [J. CHEM.EDUC., 39, 252 (1962)l on the use of hypodermic syringes to illustrate the gas laws, we have used them in our general chemistry laboratories to provide a graphic and quantitative illustration of Boyle's Law. The high cost of air-tight, 50-cc, glass syringes, however, has prompted us to substitute plastic syringes (Monoject 550-SR, Shemood Medical Industries) for this purpose. The plastio syringes consist of a polypropylene barrel snd a rubber-tipped polypropylene plunger. Considerable friction exists between the polypropylene and the rubber so that soeeid nrocedures must be followed in order to eet satisfactorv results. Results
tions; pressing down on the textbooks several times before each volume reading so that equilibrium was always approached from the same direction. The figure illustrates the type of results that can be ohtained by following these procedures. In our laboratory assignment the students weigh the piston and the textbooks on a two-pan bdsnce and then use two points an their Bayle's Law plot to caloulrtte the barometric pressure by the equation
where B is the desired barometric pressure, a is the mass of the piston, the area is the cross-sectional area of the piston, the b's and V's are the masses of the hooks and the corresponding volumes for the two points chosen for the calculation, and the 0.736 is required to convert g per cnia to mm of Hg. With the plastic syringes, values of B that were within 2 4 mm of the measured value were obtained; this compares favorably with the results obtained with glass syringes. The plastic syringes were also exsmined far use in illustrating Charles' Law. As was reported far glass syringes, measurements cannot be made at temperatures that are too divergent from room temperature because of heat leakage through the piston of the syringe. This severely limits the range of temperatures over which measnrements can he made and necessitates a rather long extrapolation of the plat of volume against temperature to arrive a t the value of absolute zero. Again, however, if the above precautions are employed in the operation of the plastic syringes, results are obtained that are comparable with the results for the glass syringes.
L. C. GROTZ
J. E. GAUERKE
Volume 48, Number 5, May 1971
/
337
