52 PRINCIPLES OF CHEMISTRY. 



or on chemical changes generally. If, for instance, the analysis of a 

 substance shows of calcium 40 per cent., of carbon 12 per cent., and 

 of oxygen 48 per cent., we have a right to assume that the molecule is 

 made up of 1 atom of calcium, 1 atom of carbon, and 3 atoms of 

 oxygen, as the atomic weights of these elements are 40, 12, and 16 

 respectively. The molecular weight in this case is 100, and the com- 

 position is expressed by the formula CaCO 3 , but the molecular weight 

 might be 200 and the correct formula Ca 2 C 2 O 6 . There are actually 

 substances which contain such multiples of atoms, as, for instance, the 

 compounds C 2 H 2 and C 6 H 6 , and as their percentage composition is 

 identical, analytical methods are insufficient to indicate the number 

 of atoms contained in these molecules. 



The second method, based on Avoggdro's law, is applicable to all 

 substances which are or can be converted into gases or vapors without 

 decomposition. Weighing equal volumes of hydrogen and of some 

 other substance in the gaseous state gives at once the data for calcu- 

 lating the molecular weight. (See page 44.) 



A third method, that of Raoult, is based upon the fact that the 

 freezing-point of a liquid is lowered to the same extent by dissolving 

 in it compounds in quantities proportional to their molecular weights. 

 For example : Water begins to solidify at 32 F. (0 C.), but by dis- 

 solving in it say 4 per cent, of its weight of a salt (the molecular weight 

 of which is known) the freezing-point is lowered, say 1 C. If, then, 

 another salt (the molecular weight of which is not known) be dissolved 

 in water, and it be found that to reduce the freezing-point 1 C. there 

 must be dissolved a quantity equal to 7 per cent, of the weight of the 

 water used then are the molecular weights of the two salts to each 

 other as is 4 to 7. 



In regard to this method of Raoult it should be stated that it is 

 applicable only to such substances as do not act chemically upon the 



QUESTIONS. 61. What are the three principal methods used for the deter- 

 mination of atomic weights ? 62. Why are chemical means not always sufficient 

 to determine atomic weights ? 63. How can the specific gravity of elements in 

 the gaseous state be used for the determination of atomic weight ? 64. Describe 

 a method of the determination of atomic weight by chemical means. 65. State 

 one of the reasons why the atomic weight of oxygen has been changed from 8 

 to 16. 66. What relation exists between atomic weight and specific heat? 

 67. State the law of Dulong and Petit. 68. Suppose the specific heat of an 

 element to be 0.1138, what will its atomic weight be ? 69. Suppose the specific 

 gravity of an elementary gas to be 14, what will its atomic weight be ? 70. Sup- 

 pose 216 grammes of an element replace 2 grammes of hydrogen in 73 grammes 

 of HC1, what will the atomic weight of the element be ? 



