CHLORINE. CALCULATIONS 149 



Density Compared with Air. We have seen (p. 85) that 

 22.4 liters of air weigh 28.95 g. Is water vapor heavier or lighter, 

 at the same temperature and pressure? The formula, H 2 0, shows 

 that 22.4 liters of steam weigh 2 X 1.008 + 16 = 18.016. Air is 

 heavier, in the ratio of 29 : 18, approximately. 



The Volume of Gas from Given Weights of Material. 



What volume of oxygen at and 760 mm. is obtained by heating 

 15 g. of potassiurn chlorate? Follow the rules given (p. 135) for 

 calculating weights: 



2KC1O 3 -> 2KC1 + 30 2 . 



Weights: 2 (39.1 + 35.46 + 3 X 16) 2 (39.1 + 35.46) 3 X 32 

 or 245.12 149.12 96 



Now, insert the volumes in the case of substances which are gases. 

 Here oxygen is the only one. Remembering that 2 (=32 g.) 

 occupies 22.4 liters, we see that the volume of the oxygen is 3 X 

 22.4 liters, or 67.2 liters. As the next step, add the data given 

 and the x in their proper places. The x here goes under the vol- 

 ume of oxygen: 



2KC10 3 -> 2KC1 + 30 2 . 



245.12 g. 149.12 g. 96 g. (or 67.2 liters) 



15 g. x 



The problem now reads thus: If 245.12 g. of potassium chlorate 

 give 67.2 liters of oxygen, 15 g. of potassium chlorate will give 

 x liters of oxygen : 



245.12 :67.2 :: 15 : x (= 4.07), 



where x } the answer, is 4.07 liters of oxygen. 



Observe that, when we use the gram-molecular volume, one 

 proportion gives the answer. It is not necessary to make two 

 steps in the calculation, by finding first the weight of the oxygen, 

 and then its volume. The beginner always tends to do this, until 

 he learns by experience that it takes twice as long to solve the 

 problem in this way and that the chance of obtaining the wrong 

 answer by making arithmetical errors is greatly increased. 



