MOLECULAR WEIGHT OF GASES 19 



also be equal, which amounts to saying that R referred to 

 the molecular quantity has the same value for all gases. 



Taking 2 kilograms of hydrogen as basis for calculating 

 this value of R, we have for atmospheric pressure and 



oC. 



P = 10333 kg/sq. m. T = 273 V = cub. m. 



since one litre of hydrogen at o and atmospheric pressure 

 weighs 0*08956 gm. Therefore 



R = 845-2. 

 Since this number happens to be approximately twice 



the value of the calorie in kilogrammetres (-T- = 423) the 



expression for the laws of Boyle, Gay-Lussac, and Avogadro 

 simplifies, approximately, to 



B. Methods for Molecular Weight Determination in 



Gases. 



i. Ordinary Process. 



The application of Avogadro's law, formerly the only 

 means of determining molecular weights, has, since the 

 introduction of the simple methods based on the theory 

 of solutions, been practically confined to Victor Meyer's 

 process 1 . 



, As in all such methods, the procedure consists essentially 

 in measuring the volume (V) of a known weight (G) as gas 

 or vapour under a definite pressure (P) and temperature 

 (T). An equal volume of hydrogen would, under these 

 circumstances, weigh 



273 PV 



0-08956 PVx-tp = 24-45^- 

 if weight, volume, pressure, and temperature are given in 



1 Berl. Ber. 30. 1926. 

 B 2 



