36 AGRICULTURAL CHEMISTRY 



of the chemical changes which take place in the soil are 

 dependent upon this element. Because of its wide distri- 

 bution in nature, it is not given such economic considera- 

 tion as are other elements, but it is one of the most im- 

 portant, and is as necessary for life as other food. 



Problem I. How many pounds of oxygen are required to com- 

 bine with 25 pounds of pure carbon ? When carbon is burned, I 

 part of C (called an atom) unites with 2 parts of O (2 atoms of O) 

 to form the compound CO 2 . This is expressed by the equation 

 C -f 2O = CO 2 . The atomic or least combining weight of carbon is 

 12 and of O is 16; one part by weight of C weighing 12 unites with 

 2 parts by weight of O, each part weighing 16 ; or 12 parts by 

 weight of C unite with 32 parts by weight of O. If the parts are 

 designated pounds then 25 pounds of C will require proportionally 

 as much O as do 12 pounds of C. This amount can be determined 

 by a simple proportion. 



C : O : : C :O 

 12 : 32 : : 25 : x. 



By solving this proportion, x, or the required amount of O to com- 

 bine with 25 pounds of C, is found to be 66 2/3. 



In the solving of chemical problems some of the most 

 common errors are : ( i ) Failure to write properly the 

 formulas of the compounds used, or the equation repre- 

 senting the chemical reaction that takes place. This 

 error causes the wrong number of parts of elements or 

 compounds to be taken in the proportion. (2) Failure 

 to make proper use of the combining weights of the ele- 

 ments. (3) Failure to combine properly the weights so 

 as to form a true proportion. It should be remembered 

 that after the writing of the equation and weights, the 

 problem becomes simply one of arithmetic. 



Problem 2. How many pounds of CO 2 are produced when 25 

 pounds of carbon are burned ? 



Problem j. How many pounds of carbon are necessary to com- 

 bine with 25 pounds of cf in forming CO 2 ? 



