136 SMITH'S INTERMEDIATE CHEMISTRY 



If the expanded equation has been prepared correctly, this final 

 statement is purely mechanical. It will be seen that only two of 

 the three quantities in the equation were really used. 



Alternative to the Sixth Step. We may also say: If 

 32.06 g. of sulphur will give 97.43 g. of zinc sulphide, 1 g. of sulphur 



will give Q ' g. (= 3.039 g.) of zinc sulphide. Then, if 1 g. of 

 o^.Uo 



sulphur gives 3.039 g. of zinc sulphide, the 100 g. of sulphur will 

 give 100 X 3.039 g. (= 303.9 g.) of zinc sulphide. 



Warnings. In solving the exercises at the end of the chapter, 

 beware of three kinds of mistakes commonly made by beginners. 



1. Conquer a tendency to say that the symbols Zn and S 

 stand for " 1 part " of zinc or of sulphur. They stand for 1 chemi- 

 cal unit, or atomic weight, or atom, in each case, that is to 

 say, for 65.37 " parts " and 32.06 " parts," respectively. 



2. Follow the rules laid down above. When one has once 

 become familiar with the art of solving such problems, running 

 through the rules takes only a few seconds. The chemist does it 

 almost unconsciously. The beginner always thinks he can ignore 

 these rules, and he fails in consequence. Writing the equation 

 in the expanded form, and then reading the problem into it are 

 absolutely essential steps. 



3. Do not read the original problem carelessly and make the 

 equation backwards, that is, with the sides reversed. If there 

 seems to be confusion somewhere, when the last steps are reached, 

 this hint will probably show the cause of the difficulty. 



Another Example. What weight of hydrogen is required to 

 reduce 45 g. of magnetic oxide of iron to metallic iron? 



Following the rules, as before, we reach the expanded equation : 



Fe 3 O 4 + 4H 2 -> 3Fe + 4H 2 O. 



3 X 55.84 + 4 X 16 8 X 1.008 3 X 55.84 4 (2 X 1.008 + 16) 



167.52 + 64 8.064 167.52 4 X 18.016 



231.52 8.064 167.52 72.064 



45 g. x 



