STUDY OF THE ACTIVE PART OF THE AIR 



135 



wool on the balance pan, weigh. Then add about one gram of powdered 

 potassium chlorate, and weigh again. Put the plug of asbestos loosely in 

 the top of the test-tube, and connect to the apparatus. Heat gently till 

 water ceases to be displaced. 

 Allow the test-tube to cool and 

 then weigh. Also note the vol- 

 ume of water in the cylinder. 



From the results calculate the 

 weight and volume of oxygen 

 given off from one gram of 

 potassium chlorate. 



Density of oxygen. By 

 weighing a quantity of potas- 

 sium chlorate, before and after 

 heating, and measuring the 

 volume of gas given off the 

 weight of a certain volume of 

 oxygen is found, and from this 

 can be calculated the weight 

 of 1000 c.c. Careful experiments show that one litre of oxygen weighs 

 1-43 gm. at standard temperature and pressure (N.T.P. =0 C. and 

 760 mm.). As a litre of air weighs 1'293 gm., oxygen is slightly 

 heavier than air, and therefore nitrogen is slightly less dense than air. 



FIG. 97. Displacement bottle. 



QUESTIONS ON CHAPTER XXI. 



98. How is oxygen usually prepared ? Make a sketch of the apparatus 

 you would employ, and describe three experiments illustrating its pro- 

 perties you have made with the gas. 



09. Indrcate four methods of preparing oxygen : point out the advantages 

 and disadvantages of the various methods. 



100. Why was oxygen gas so called ? Who gave the gas this name and 

 what experiments led him so to name it ? 



101. You are given some mercuric oxide and a jar filled with nitrogen 

 standing over water ; state exactly how you would proceed to make a mixture 

 having the approximate composition of air. 



102. Sketch an apparatus suitable for obtaining a number of jars of inactive 

 air. How would you satisfy yourself that the gas obtained by any two 

 selected methods is the same ? 



103. Chlorate of potash loses 39' 15 per cent, of its weight on being heated. 

 What volume of gas would you obtain by heating 3 grams of the chlorate ? 

 Sketch the apparatus you would use. No description of the experiment 

 need be given. One litre of oxygen (N.T.P.) weighs 1'43 grams. 



