Verification of Avogadro's Law A lecture demonstration has been developed which provides a MINERAL rather dramatic verification of Avogadro's law. The demonstraOIL tion, which is suitable for high school and freshman chemistry courses, also illustrates Gay-Lussac's law of combining volumes. The demonstration utilizes the simple reaction between gaseous ammonia and gaseous hydrogen chloride which proceeds to completion a t room temperature yielding an essentially nonvolatile product, ammonium chloride. NHB HCL NHICI. Therefore, if stoichiometrie amounts of the two gases are mixed in a closed system with no other gases present the pressure within the system will drop to zero since no gases remain. Avogadro's law states that equal volumes of different gases under the same conditions of temperature and pressure contain the same number of molecules. Therefore, according t o Avogadro's law equal volumes of ammonia and hydrogen chloride should be stoichiometric amounts of these gases with respect to the above reaction. This prediction may he tested using the apparatus shown in the figure. The round bottom flasks can be filled by simply disconnecting the pieces of rubber tubing a t the appropriate places and flushing the flasks with the respective gases. The manometer can he evacuated prior to connection to the flasks to eliminate air from the system. However, if the air is not removed, one can ac300 ML. count for its presence by knowing the volume of the tube between the mercury and the adjacent flask. After assembling the apparatus the gases can he mixed by displacing the ammonia with mineral oil, in which ammonia is only very slightly soluble. As the ammonia is displaced a dense white smoke of ammonium chloride is produced in the flask containing hydrogen chloride. After the transfer of ammonia is complete the pressure of the system can be measured using the manometer together with a knowledge of atmospheric pressure. There should be no pressure due to the presence of any ammonia or hydrogen chloride. However, if air was not removed from the manometer a small pressure will exist. One can correct for this easily. For example, if the volume of air between the hydrogen chloride flask and the mercury was 10 ml prior to mixing the gases, then a t conclusion of the reaction the air will have expanded to the volume of the hydrogen chloride flask and accordingly the pressure due to the air will have dropped to 2 cm of mercury (assuming atmospheric pressure t o be 76cm). This should he the final pressure.
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M. D. Alexander New Mexico State University Las Cruces, 88003
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Journal of Chemical Education
