The AVOGADRO NUMBER JOSEPH E. JOHNSTON Christopher Columbus High School, Bronx, New York
HE Avogadro Number is one of the fundamentally important constants of both chemistry and physics. Most of us are familiar with its importance in relation to the gas laws and in the determination of innumerable atomic and molecular quantities. Yet, of the many chemists and physicists I have met, I have still to find one who in his studies in science has made a determination of this enormous figure. In fact, it is the very enormity of this quantity which has condemned it to almost solitary confinement in the innermost shrines of the advanced research laboratories. Once in a great while, as in the case of another universal constant, the speed of light, someone refines it a little or devises another method for its determination and then it is again retired for an indeterminate period. In the course of discussions on the Kinetic-Molecular Theory and the gas laws, I have occasion to show the students a replica of the gram-molecular volume. Invariably someone asks how many molecules i t contains. When informed, their brows knit somewhat skeptically as they endeavor to comprehend this incomprehensible figure. It improves matters only slightly when you apprise them of the fact that if every individual on the eartl-some two billion-were to start counting a t the rate of ten a second without taking time out for anything, it would require a million years to count an equivalent number of pennies. But when I go a step further and inform them that certain members of the class can count that number of molecules in less than twenty minutes, they are frankly dubious. Yet such is the case. Some time ago one of my senior students with a splendid background of mathematics and physics asked me what apparatus i t would require to make an assault on N. He had been looking over a copy of Hoag's "Electron Physics" and had come across a passage wherein it is suggested that the constant can be obtained by electrolysis. His enthusiasm infected me and working together the following method was devised and tried out with truly astonishing results. The equipment required is well within the reach of the average high school. The term electrolysis immediately suggests the Hoffman Apparatus. We were fortunate enough to have one calibrated a t 20°C., which may be had from any scientific equipment company. We used the instrument without further calibration since we felt that with our limited equipment we could not improve on this product of a reliable concern. Our source of direct current was an ordinary six-volt storage battery, and the flow of current was measured bv an ammeter selected from a
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group of twenty laboratory ammeters because it was the mean when they were all tested. Again taking advantage of the law of averages, we selected a centigrade thermometer and hung our mercury barometer with its vernier scale nearby. The physical training department of the school loaned us a stop-watch for
LEGEND A. B C. D. E. F.
Ammeter Storage Battery Electric Clock Siphon Electrolysis Apparatus Dipole Switch G. Beaker H. Rubber Tube
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which we later substituted an electric clock with a second hand, which made it possible to synchronize the starting of the electrolysis and the timing. This was a decided improvement, since it enabled us to read rather accurately to a fifth of a second without having to wony about the exact starting and stopping of the watch. The final hook-up appears in the accompanying diagram. Between fifteen and twenty-five minutes were
found sufficient to yield a working quantity of gas a t an amperage between 0.1 and 0.25. Since polarization causes a slight fluctuation in the current flow we took meter readings a t minute intervals and used the mean. Before starting the actual determination the acidulated water in the apparatus should be saturated with hydrogen by permitting the passage of the current for about fifteen minutes. After this is done, the apparatus is ready for the experiment. After a sufficient quantity of hydrogen has accumulated it is necessary to equalize the pressure in all tubes before a correct reading can be made. This is done by attaching a rubber tube (H) to the oxygen outlet and forcing the water down to the hydrogen level by gently using the breath. When this is done water in the reservoir may be drawn off, as indicated, by means of the siphon (D), if the fact, that the water is slightly acidulated with sulfuric acid is kept in mind. It is best to siphon off slightly more water than necessary and then equalize by carefully adding water from a dropper or pipet. The quantity of hydrogen collected may now be read and the usual corrections for temperature and pressure made, as will be indicated. This value with the average amperage and the time in seconds are the only facts determined experimentally. With these are used the value of the charge on the electron as determined by Millikan in his famous oil-drop experiment (1.591 X 10-I= coulombs), the value of the gram-molecular volume in cubic centimeters (22,400 cc.) and the uumher of atoms in the hydrogen molecule which is two. From this information and with a prayer that whatever errorsthere might be would cancel out, we were consistently able to get results ranging from 5.87 to 6.26 X loz3. Our best readings were 6.04 and 6.08 X loz3. In brief, the Avogadro Number is obtained as follows. The product of the time in seconds and the average amperage gives the number of coulombs required to liberate the corrected volume of hydrogen in cubic centimeters. Dividing the former by the latter and multiplying by 22,400 we obtain thenumber of coulombs it takes to free a gram-molecular volume of the gas. When this figure is divided by the charge on one electron
SPECIAL RESEARCH CONFERENCES OF THE CHEMISTRY SECTION OF THE A. A. A. S. This conference will be held at Gibson Island, Maryland, dming July 10-14, July 17-21, and July 24-28. The meeti?gs will begin at ten o'clock, wlth one or two formal papers outlinmg the fields of research, and directing attention to the unsolved problems. It is advisable to make reservations in advance. For reservations, or further information, write to the secretary, Neil E. Gordon, Central College. Fayette, Missouri. PROGRAM
HENRY GILIAN.Iova State College, General Chairman A. Resinous Polymns, L. ,B?ep?q*uo, Honorary Chairman; H o w a n L. Berrosa, Chairmr July 10. L. H. Backrana, l n r r o r Howard L. Bender, "T particles as to size and LmaapeThomas Midgley, Jr., Leader of diseur3ion of preceding paper -t 7:00 F.W. in honor of L. H. Baekeleland July 11. R. H Kienle. "The Infusible Resinous State'' " " " . u u . . " .u. ."o..x.:>;..S. S Kis tier, .,r.--2;.:--. July 12. S. D. Do ugh$, "The Resinous Vinyl Compounds" I v y AllelI, "Polysty'ene" July 13. C. S. Ma~ruel,.'The Structure of Some Vinyl Polymers" R. B Ea rnes .Speetrosmmand Resin Structure" July 14. G. M. K line.'"~lectronlc Structure and the Behavior of Cellulope C
