FEBRUARY. 1937 MOLECULAR WEIGHT OF CARBON DIOXIDE DRULEY PARKER
AND
ROBERT URMSTON
Shortridge High School, Indianapolis, Indiana
THE purpose of this experiment is to determine the molecular weight of a water-soluble gas by making use of the Avogadro volume. It may be used as a class demonstration or as a special experiment for those students who are ahead of the class or wish to do extra experiments. These students are sometimes a problem to an overworked teacher. The laboratory part of this experiment was performed by such a student, Mr. Robert Urmston. The method is an evolution method and consists in evolving a water-soluble gas, carbon dioxide, which displaces an equal volume of air, which in turn displaces an equal volume of water. The carbon dioxide is obtained by heating a carbonate and the weight of the carbon dioxide is obtained by weighing the tube containing the carbonate before and after heating. The volume of the gas evolved is obtained by weighing the water displaced. An attempt was made to find a carbouate which would not give off water upon heating and which would decompose at a moderate temperature. After trying several carbonates the couclusion was reached that this was not feasible. Those carbonates which evolve only carbon dioxide decompose a t temperatures close to the softening point of glass, while thosethat decompose at lower temperatures seem to be hydrated carbonates or mixtures of carbonates and hydroxides or are basic carbonates. Then it was reasoned that possibly the water was evolved at a lower temperature than the carbon dioxide, thus making it possible to first prepare the carbonate by driving off the water before using the substance in the experiment. To test this idea a small electric furnace was made, consisting of a nichrome wire wound around a large pyrex test-tube and connected in series with a variable resistance. The carbonate, a thermometer, and a lead-off tube (which passed into some calcium hydroxide solution) were placed in a smaller tube which in turn was inserted into the larger tube wound with wire. In the case of manganese carbouate, moisture came off continually from 230' up to the softening point of glass, while the carbon dioxide was first evolved a t approximately 243'. Consequently, the water could not be driven off without the carbon dioxide coming with it.
Referring to the order of events, the drying tube and test-tube plus the carbouate are weighed and then the apparatus is set up according to the diagram. The temperature is obtained by placing a thermometer in the first flask and the barometer is read. The apparatus is tested for leaks by blowing in the exit tube and then letting water siphon out. If any leaks are present, the water will continue to flow out of the siphon. The level of water in the beaker is made the same as that in the flask by raising the beaker, and after time has elapsed for the pressures to equalize, the pinch clamp is closed. The water is emptied from the beaker and the beaker is weighed, replaced under the exit tube, and the clamp opened. Heat the carbonate in the tube until approximately 1500 cc. of water has siphoned over into the flask. To obtain this amount of carbon dioxide about ten grams of manganese carbonate will have to be heated. When a goodly portion of water has been siphoned over, stop heating the tube, and let the apparatus return to room temperature. Equalize the levels of water as before, and close the clamp. Weigh the beaker and water and thus obtain the volume of the carbon dioxide. Again, weigh the tube containing the carbonate and also the drying tube, thus obtaining the weight of the carbon dioxide.
W t . d w i n g WI. d r ~ i x d tube, test tub#, trrc WI. (8.) Vor. ( ~ 6 . ) P Mol. tube. MnCO. tuba, MnO COI CO. (nm.) Tan*. wt. 93.189 94.704 80.083 79.846
91.522 92.429 77.875 77.839
1.887 2.276 2.208 2.007
980 1309 1245 1143
748 731 740 739
21' 17'
42.8 43.1
21'
44.0
23-
43.8
