84 SMITH'S INTERMEDIATE CHEMISTRY 



The numoer of molecules actually contained in the G.M.V. 

 has been determined by several independent methods, the agree- 

 ment of the results obtained furnishing very strong support for 

 the validity of the molecular hypothesis. The value at present 

 accepted as most accurate is that of Millikan, 6.06 X 10 23 *. It 

 is of importance that the student should obtain some idea of the 

 significance of this stupendous number, in order to appreciate 

 the more detailed discussion of the properties of gases in the follow- 

 ing sections (p. 88-93). Imagine a G.M.V. of a gas at and 760 

 mm. to be divided equally among the inhabitants of the United 

 States (say 110,000,000). Each person would receive about one- 

 fifth of a cubic millimeter as his share. Imagine, further, that 

 the market price of a million molecules of this gas was one cent. 

 Few of the recipients would think it worth while to cash in their 

 microscopic sample; those doing so, however, would benefit to 

 the extent of over 50 million dollars ! 



Molecular Weight, Measurement of. The molecular 

 weight is measured by weighing any convenient volume of the gas 

 (say 200 c.c.), and calculating by proportion the weight of 22.41. 

 If the gas or vapor was not measured at and 760 mm., the 

 measured volume must be reduced by rule to standard conditions 

 before the weight of 22.4 1. is calculated. f 



* This is the method used in physics and chemistry to express numbers 

 so large as to be cumbrous and incomprehensible if written in the ordinary 

 way. It means that the unit is to be followed by the number of zeros 

 indicated by the exponent. In this case, the quantity written out at length 

 would be 606,000,000,000,000,000,000,000. 



f In practice, owing to the fact that Boyle's and Charles' laws do not 

 describe the behavior of any known gas exactly (they apply only to a "perfect" 

 gas), certain additional, small corrections have to be applied when very pre- 

 cise values are required. It may be noted that, in order to make the funda- 

 mental significance of the numerical data presented in the preceding pages 

 immediately intelligible to the beginner, all of the gases so far considered have 

 been assumed to be "perfect." The actual experimental values differ only 

 very slightly from those given. 



