1 8 AGRICULTURAL CHEMISTRY 



follows : 2 atoms of H -+- I atom of O equal i molecule of water, 

 or 2H -(- O = H 2 O. An atom of O weighs 16 times as much as 

 an atom of H. Two atoms of H and i atom of O weigh 18 times as 

 much as an atom of H. The molecular weight of water is 18. 

 Sixteen of these 18 parts, by weight, are O, or 16/18 are oxygen, 

 which is 88.88 per cent.; 2/18, or 11.12 per cent, being H. In the 

 production of water, H and O always unite in this proportion. If, 

 for example, 20 grams of O and 2 grams of H were brought to- 

 gether, only 1 6 grams of O would enter into chemical combination 

 with the 2 grams of H, and 4 grams of O would be left uncombined. 

 The amount of H required to combine with 20 grams of O would 

 be obtained from the following proportion, 2 : 16 : : x : 20, or 

 :r 2.5 grams of H. 



Problem 2. (i) Calculate the per cent, by weight of C and O in 

 CO 2 . (2) Calculate the per cent, of Fe and O in Fe 2 O 3 . (3) 

 Calculate the per cent, of O in KC1O 3 . 



