LABORATORY APPARATUS FOR ILLUSTRATING GRAHAM'S EFFUSION LAW PAUL
EMILWENAAS, MONTANA STATE COLLEGE, BOZEMAN, MONTANA
The construction and the use of a n apparatus zllustrating Graham's Effusion Law i s described. It can be easily constructed of ordinary laboratory epuipment. Its adz'antqe lies in its ease of replacement offarts in case of breakage and the elimination o f corrections necessary i n some types of apparatus. Use of the apparatus by students in physical chemistry at Montana State College, Bozeman, Montana, has shorn that the apparatus can be easily manipulated, the experimental results of molecular weights obtained checking in some cases very closely with the true z'alues.
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An apparatus illustrating Graham's Effusion Law which can he easily constructed of ordinaw laboratory equipment has been successfully used in the physical chemistry laboratory of the chemistry department a t M o n t a n a State College. Its advantage lies in its ease of replacement of parts in case of breakage and the elimination of corrections for moisture necessary in some types of apparatus. I t consists of a filter flask, A, equipped with a one-hole rubber stopper and a 50-cc. pipet. To the pipet is attached a two-hole rubber stopper. Over the hole in the stopper connected with the pipet is sealed with sealing wax a thin metal plate, B, with a very small opening. Sheet aluminum or sheet copper can be very conveniently used for this purpose. A needle which has been filed to give a long narrow point is used to 0 make the opening in the plate. In place of the metal plate, a piece of glass tubing drawn to a small opening on one end may be inserted in the same hole in the stopper. This, however, must be so constructed that the length of the narrowed 2257
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JOURNAL OF CHEMICAL KDUCATION
Novn~mn,1931
part is the minimum possible, in order to eliminate capillary action. An inverted Goochfilter, C, is placed over the rubber stopper. The opening a t H must be made small enough to maintain a slight positive pressure in C, either by drawing out II or by placing a piece of small rubber tubing over the top of the inverted filter. A piece of glass tubing connects the second hole in the stopper with an Erlenmeyer flask, D, containing sulfuric acid to dry the gas used. Good connections must be used throughout to eliminate errors due to leaks. The gas from the supply streams through the drying flask and the Gooch filter to displace all of the air in the apparatus. During this time the sulfuric acid in the filter flask is forced up the pipet nearly to the lip and suddenly released by opening the pinch clamp, E. This is repeated until all of the air is out of the apparatus; then it is repeated and the time necessary for the acid level to pass from F to G is measured with a stop-watch. Use of the apparatus by students showed that the flow of gas should be so regulated that it enters D a t the rate of two bubbles per second. A greater rate of flow is, however, not objectionable as long as the opening H is large enough to equalize the pressure inside the Gooch filter and the pressure of the atmosphere. A too-small rate of flow will allow air to enter H, contaminating the gas being used and introducing errors in the experiment. It is recommended that the opening in the metal plate or in the glass tube be so adjusted that the time for oxygen is between 80 and 110 seconds. With oxygen used as a standard, the molecular weights of nitrogen, carbon dioxide, air, and hydrogen were observed, and agreed with the true molecular weights within 0.75 to 2.09%.
