DETERMINATION OF ATOMIC WEIGHTS. 109 



weight of its atom 1. Hydrogen is usually chosen for comparison 

 with other gases. Suppose it is desired to find the molecular weight 

 of oxygen. One liter of oxygen at C. and 760 mm. of pressure 

 weighs 1.429 grammes, one liter of hydrogen under the same conditions 

 weighs 0.08987 gramme. Hence by the proportion, 



0.08987:1.429 : : 2:x, 



x = 1 .429 X 2 -5- 0.08987 = 31.8, 



that is, the molecular weight of oxygen is 31.8, or the molecule is 15.9 

 times heavier than the molecule of hydrogen. 



If we call the density of hydrogen 1, and refer the densities of 

 other gases to this standard, then the figures indicate how many times 

 heavier the gases are than hydrogen under like conditions, or, what 

 comes to the same thing, how many times heavier the molecules of the 

 gases are than the molecule of hydrogen. Hence, a simple rule for 

 finding molecular weight is to multiply the density of a gas on the 

 hydrogen basis by 2. 



Conversely, if we know the molecular weights of two gases, and 

 the density of one of them, we can calculate the density of the other 

 gas. The density of any gas is equal to the density of hydrogen 

 multiplied by half the molecular weight of the gas. 



A third method, that of Raoult, is based upok 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 



QUESTIONS. What are the three principal methods used for the determina- 

 tion of atomic weights ? Why are chemical means not always sufficient to 

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

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

 method of the determination of atomic weight by chemical means. State one 

 of the reasons why the atomic weight of oxygen has been changed from 8 to 16. 

 What relation exists between atomic weight and specific heat? State the law 

 of Dulong and Petit. Suppose the specific heat of an element to be 0.1138, 

 what will its atomic weight be? Suppose the specific gravity of an elementary 

 gas to be 14, what will its atomic weight be? Suppose 214.24 grammes of an 

 element replace 2 grammes of hydrogen in 72.36 grammes of HC1, what will 

 the atomic weight of the element be? 



