OXYGEN. 139 



barium dioxide decomposes into barium oxide and oxygen, which latter is 

 pumped away and stored in tanks. Oxygen of about 96 per cent, is obtained. 

 The changes taking place in the two stages are represented thus : 



BaO + O = BaO 2 ; 



This process is a good example of the kind of change known as Reversible 

 Action (see page 114). When the dioxide is exhausted, the process is re- 

 peated. One kilogram of barium oxide yields about ten liters of oxygen at a 

 single operation. 



The quantity of oxygen liberated from a given quantity of a substance may 

 be easily calculated from the atomic and molecular weights of the substance 

 or substances suffering decomposition. For instance : 100 pounds of oxygen 

 may be obtained from how many pounds of potassium chlorate, or from how 

 many pounds of manganese dioxide? (See page 100.) 



The molecular weight of potassium chlorate is found by adding together 

 the weights of 1 atom of potassium = 38.86 + 1 atom of chlorine 35.18 + 3 

 atoms of oxygen = 47.64; total = 121.68. Every 121.68 parts by weight of 

 potassium chlorate liberate the weight of 3 atoms, or 47.64 parts by weight, of 

 oxygen. If 47.64 are obtained from 121.68, 100 are obtained from 255.4. 



47.64 : 121.68 : : 100 : x 



x = 255.4. 



In a similar manner, it will be found that 815.7 pounds of manganese dioxide 

 are necessary to produce 100 pounds of oxygen. Mn0 2 = 54.6 -4- 31.76 = 86.36. 

 3Mn0 2 = 3 X 86.36 = 259.08. Every 259.08 parts furnish 2 x 15.88 = 31.76 

 parts of oxygen. 



31.76 : 259.08 : : 100 : x 



* = 815.7, 



The density of a gas is the weight of 1 liter. To find what volume corre- 

 sponds to a given weight of a gas, divide the weight by the density. The den- 

 sity of oxygen is 1.429 grammes in 1 liter at C. and 760 mm. pressure. 

 Hence, under these conditions, 100 grammes of oxygen would measure 100 -*- 

 1.429 = 69.979 liters. (For method of calculating gas volumes under other than 

 standard conditions of temperature and pressure, see article on Gas Analysis.) 



The densities of gases are generally given in books, but they can be calcu- 

 lated, if the molecular weights of the gases are known. The relation between 

 densities and molecular weights of gases is discussed on page 108. The density 

 of any gas is equal to the density of hydrogen multiplied by one-half the 

 molecular weight of the gas ; 1 liter of hydrogen at C. and 760 mm. pressure 

 weighs 0.08987 gramme, the molecular weight of oxygen is 31.76; hence 1 liter 

 of oxygen weighs 0.08987 X 15.88 = 1.427 grammes. 



Experiment 1. Generate oxygen by heating a small quantity (about 

 grammes) of potassium chlorate in a dry flask of about 100 c.c. capacity, to 

 which, by means of a perforated cork, a bent glass tube has been attached, 

 which leads under the surface of water contained in a dish (Fig. 37). Collect 

 the gas by placing over the delivery-tube large test-tubes (or other suitable ves- 



