134 



not know even the first place accurately. Hence 8.427 is just 

 as likely to be the exact value as the longer number. In the most 

 exact calculation we round the number off, usually, at the second 

 decimal. For rougher purposes the first decimal place is suffi- 

 cient. 



The Formula- Weight. The sum of the weights of the 

 constituents indicated in the formula is called the formula-weight. 

 For silver nitrate this has the value, 107.88 + 14.008 + 48, or 

 169.89 (Query: Why not 169.888?). 



If the substance is a gas, or is easily volatile, the formula-weight 

 will be also the molecular weight. Thus, acetylene gas (used in 

 lighting) has the formula C 2 H 2 . The composition is 2 = 2 X 

 12.005 or 24.010 parts of carbon and H 2 = 2 X 1.008 or 2.016 

 parts of hydrogen. The molecular weight is 24.010 + 2.016 

 = 26.026. Again, hydrogen peroxide (used in medicine), a li- 

 quid, is decomposed when boiled, but it dissolves in water and 

 depresses the freezing-point. Its molecular weight has therefore 

 been determined (p. 119) and is H 2 2 . The molecular weight on 

 which this formula is based is 2.016 + 32, or 34.016. 



When the substance is neither volatile nor soluble, the simplest 

 formula is always used, and therefore only the formula-weight 

 can be ascertained. 



To Find the Percentage Composition. In silver nitrate 

 the proportions are 107.88 of silver, 14.008 of nitrogen, and 48 of 

 oxygen in a total of 169.89. In one hundred parts, the silver will 



be j^|| X 100, or 63.50; the nitrogen ^| x 100, or 8.25, 



A Q 



and the oxygen 1Qg gQ X 100 = 28.25. 



The same results may be obtained by the rule of proportion. 

 Thus, for the silver, 169.89 : 107.88 :: 100 : x where x is the per- 

 centage of silver. 



