OXYGEN GAS B. A. FIEKERS, S. J. College of the Holy Cross, Worcester, Massachusetts
A VERY convenient source of oxygen is provided by the "Oxybomb."' I t is a small, 2'/2 by 5/rinch cartridge of compressed oxygen that resembles a carbon dioxide source, such as the "Sparklet," for carbonating water in a siphon bottle. The cartridge of oxygen comes fitted with a threaded collar so that a specially designed needle valve can be attached to it for puncturing the soft metal gas seal and for controlling the flow of gas. It is the purpose of this paper to show how this oxygen source can be used for determining the density of oxygen gas in laboratory or lecture. All currently popular laboratory manuals describe the chemical generation of oxygen, one of them recommending the thermal decomposition of lead peroxide ( 1 ) ; all of the others, such as Holmes ( g ) , the thermal and catalytic decomposition of potassium chlorate. Possibly the method proposed in this paper has a pedagogical advantage in separating the physical aspects of the gas laws from the chemical preparation of the gas. It would give the student fewer principles to learn in a unit exercise and this might appeal to some instructors. Further, this exercise consumes but little time. I t may thus be of value to instructors who are seeking alternative exercises to fit given time schedules. At any rate, this experiment can be used for demonstration to groups in the laboratory alongside the traditional counterparts which give equal emphasis to the chemical and the physical principles involved.
backward until, at any backturn, gas bubbles into the bottle. As soon as the seal is known to be punctured cut down the flow of gas to about one bubble per second. Allow the gas to replace the water in the bottle until the same water level in the trough and bottle is reached. Then disconnect and weigh the cartridge again and get the weight of the released oxygen by the difference between this and the original weight of the cartridge (AW). Slip the mouth of the inverted bottle over a glass plate under water, maintaining equal water levels as far as possible. Then invert the bottle briskly so that it neither gains nor loses water (practice). Now measure the volume of oxygen in liters (V,) by filling the gas bottle to the brim with water from a graduated cylinder. Take the temperature (Tabs.) of the water. Get the barometric pressure in millimeters of mercury, making instrumental corrections, and correct for the vapor pressure of water as well (P,,,). Calculate the density (d) of oxygen gas a t standard conditions. d = (760 X Tab.. X AW)/(273 X PGo.,X VI)
Multiply the density by 22.4 and see if it checks the molecular weight of oxygen. Sample results are given in the table. Sample Results Deln.
A
B
PROCEDURE
The follolling procedure is recommended for student exercises in determining the density of oxygen and checking the molecular weight of a given sample. Set up a one-liter gas bottle in a pneumatic trough. Fill the bottle with water in the usual way and avoid I n this experiment if the needle is driveu all the way the presence of air bubbles in the bottle. Provide a in and then withdrawn from the gas seal, it cannot gas inlet with a suitable length of rubber tubing which always be closed tightly again and gas is apt t o leak will fit the needle valve snugly and yet allow for ready during the second weighing. Further, if the gas is connection and disconnection. Attach the needle valve allowed to flow too rapidly into the bottle, the results tightly to the cartridge of gas, taking care not to punc- may come out too high. I t seems that pressure and ture the gas seal. Weigh the cartridge and valve, consequent leakage in the head of the valve can be without the rubber tubing, on a triple beam balance minimized by slowing down the flow of gas as directed. and estimate the weight t o the third decimal place in The valve is obviously not designed for quantitative grams. Then connect the rubber tubing to the valve. applications. I n order to puncture the seal, first turn the valveLecture or group demonstrations can be speeded up head down loosely until contact between needle and by weighing the cartridge beforehand; by releasing the seal is established. Then advance the screw succes- oxygen to a predetermined volume so that the corresively about half a turn forward and a quarter turn sponding cartridge weight can be anticipated by calcula'The "Oxybomb" and noedlevalve are marketed by A. Daigger tion; and then by weighing the cartridge with the weights thus set beforehand. It is essential only t o & Co., 159 West Kineie St., Chicago 11, Ill.
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show that a balance is strnrk before the weight data Other gases are commercially available in cartridge are taken. form: among them, carbon dioxide, as already menArcording to most laboratory manuals, such as tioned; nitrous oxide for whipping cream; and isobuSisler and Stewart (S),the chemically generated gas is tane, recently introduced for cigarette lighters. Bepassed into a filled wash bottle in such wise that the cause of their solubility in water, they are not suitable displaced water is ejected. The volume of the gas materials for this experiment. One would need the is taken as the volume of the ejected water. A cart- services of a good mechanic t o construct safe valves for ridge source of gas can also be used with this technique. these before they could be pressed into general lahoraAccording to a few laboratory manuals (1, 4), on tory service. It is hoped that a large number of the other hand, the traditional method of using the common laboratory gases will be made available in pneumatic trough is employed. I n such instances, the some such form. amount of chemical used has to be planned carefully so that the volume of gas will not exceed the capacity LITERATURE CITED of the container. Further, both the oxygen source (1) NECHAMKIN, H., "Laboratory Problems in General Chemistry," Thomas Y. Crowell Co., New York, 1953, pp. 27-31. and the catalyst have to he dried. H. N., ''Laboratory Manual of General Chemistry," Oxygen is the gas chosen in all laboratory manuals (2) HOLMES, 5th ed., Macmillan Co., New York, 1949, pp. 23-4. that list the experiment. The procedure here suggested (3) SISLER,H. H., AND J. J. STEWART, "Systematic Laboratory may eventually make possible a choice of diverse gases. Course in Inorganic Chemistry," Macmillan, New York, 1950, pp. 55-7. The experimental objective given by most of the AND F. B. SCHIRMER, JR., laboratory manuals is the determination of the density (4) HOPKINS,B. S., T. MOELLER, "Laboratory and Classroom Exercises in General Chemisof the gas. One ( 1 ) laboratory manual chooses to try," D. C. Heath and Co., Boston, 1946, pp. 37-8. evaluate the gas constant R; another (5) evaluates the (5) RAP, F. E.,"Experimental General Chemistry," Lippincott, molecular weight of oxygen. Chicago, 1947, pp. 32-3.
