a rubber tube, which can be closed by the screw clamp C. Disconnect the bottles A and B at D and fill the bottle B and the exit tube with water, as in § 31. Weigh out accurately about 1 g. of zinc; then remove the cork from bottle A and transfer the zinc to the bottle. Also add to the bottle 1 drop of a solution of copper sulfate. Close the screw clamp C, and nearly fill the funnel with a dilute solution of sulfuric acid prepared as directed in § 23. Col OE B F- A Place a beaker under the tube F and open the screw clamp until both the rubber and glass tubes are completely filled with the dilute acid; then quickly close the clamp. Connect the apparatus just as shown in the figure, care being taken to make the joints air-tight. Now adjust the pressure of the air inside the bottles, as directed in § 31. After opening the clamp E, partially open the clamp C, and allow a few drops of the acid to flow into the bottle A. As the hydrogen is evolved it forces the water from bottle B into the beaker. More acid is added from time to time. After the zinc has all dissolved and the apparatus has acquired the room temperature, again adjust the pressure of the gas within the bottles and close the clamp E. Insert the values in the following table: Weight of zinc taken. Volume of water forced into the beaker Volume of liquid left in bottle A . Volume of hydrogen liberated. Temperature. Barometric reading FIG. 27 Reduce to standard conditions the volume of the hydrogen obtained and compare it with the theoretical results. What sources of error are involved in the experiment? CHAPTER VI NITROGEN AND THE ATMOSPHERE 50. The preparation and properties of nitrogen. (On account of its great affinity for oxygen, phosphorus must be kept and handled under water. Never bring the dry substance in contact with the skin, as it may ignite and cause a serious burn.) a. Cover the bottom of a pneumatic trough with water to a depth of 2 or 3 cm. Float on the water a porcelain crucible containing a small piece of phosphorus. Ignite the phosphorus by touching it with the hot end of a wire or file, and quickly invert over the crucible a large beaker or widemouthed bottle, being careful to keep the rim of the beaker below the surface of the water. The white fumes formed have the composition P2O. Leave the beaker in position until the fumes have entirely disappeared. Note that the water has risen in the beaker. Explain. Adjust the beaker or the water in the trough so that the level of the liquid inside and outside of the beaker is the same; then cover the beaker with a glass plate and turn it into an upright position. Test the gas with a burning splint. b. In a 250-cc. flask place a mixture of 3 g. of ammonium chloride and 6 g. of sodium nitrite, and add 20 cc. of water. Provide the flask with a cork (one-hole) and delivery tube, so that the gas evolved may be collected over water as in the case of oxygen and hydrogen. Have at hand a vessel of cold water so that the flask may be cooled by lowering it into the water in case the action becomes too violent. Clamp the flask on a ring stand and apply a very gentle heat, moving the burner about with the hand. As soon as the action begins, withdraw the burner. After the air has been expelled from the apparatus fill two or three bottles (250-cc.) with the gas. If the action becomes too violent, immerse the flask in cold water. The reaction which takes place is expressed in the following equation: NH,C1+ NaNO2 = NH,NO, + NaCl 2 2 The ammonium nitrite then decomposes into water and nitrogen (R). (The symbol (R) indicates that the equation for the reaction is to be written.) Note the physical properties of the gas. Test with a burning splint. 51. Determination of the relative volumes of nitrogen and of oxygen in the air. This determination may be made by bringing in contact with a definite volume of air a liquid which not only absorbs the oxygen but in doing so flows into the tube which contains the air and fills a space equal to that previously occupied by the oxygen. The volume of this liquid can be easily measured, and in this way the volume of the absorbed oxygen may be ascertained. The solution used to absorb the oxygen soon loses its strength on exposure to the air; hence the experiment must be performed rapidly. Before preparing the solution the student should practice the manipulations involved in the experiment. B FIG. 28 Prepare an apparatus according to Fig. 28. In this apparatus C represents a test tube about 15 cm. in length (use the hardglass tube employed in the preparation of oxygen). The tube is fitted with a two-hole rubber stopper. Through one of the holes is fitted a glass tube, which, together with the accompanying rubber tube and funnel, is the same as was used to introduce the acid into the bottle A (Fig. 27). The remaining hole in the stopper is closed with a glass rod. Notice that 1 From Cooley's "Laboratory Studies." |