HYDROCHLORIC ACID. CALCULATIONS 135 



Calculations by Use of Equations. We frequently desire 

 to know what weight of a product can be obtained from a given 

 weight of the necessary materials. For example, what weight of 

 zinc sulphide can be made with 100 g. of sulphur? It is under- 

 stood, of course, that the necessary zinc is available. 



In such calculations mistakes are easily made. The follow- 

 ing rules must be strictly followed: 



First, write down the equation: 



Zn + S -> ZnS. 



Second, place beneath each formula the weight for which it 

 stands: 



Zn + S -* ZnS. 

 65.37 32.06 97.43 



Third, read the whole statement. In this case it reads: 65.37 

 parts by weight of zinc combine with 32.06 parts of sulphur to 

 give 97.43 parts of zinc sulphide. 



Fourth, re-read the original problem. Then place the amount 

 given in the problem (100 g. of sulphur) under the formula of 

 the substance concerned. Then observe that the problem asks 

 " What weight of zinc sulphide?" and place an interrogation 

 point or an x under the formula of that substance: 



Zn + S -> ZnS 

 65.37 32.06 97.43 

 100 ? or x 



Fifth, read the problem as it now appears in this expanded 

 equation: 32.06 g. of sulphur will give 97.43 g. of zinc sulphide, 

 therefore 100 g. of sulphur will give x g. of zinc sulphate. 



Sixth. The answer may be now obtained by stating the pro- 

 portion in the same order: 



32.06 : 97.43 :: 100 : x (= 303.9). 



