Gram-Molecular Volume of Gases 103 



FAMILIARIZING CHEMISTRY STUDENTS WITH THE 

 GRAM-MOLECULAR VOLUME OF GASES. 



W. M. Blanchard, DePauw University. 



Most teachers of general chemistry have doubtless found students 

 slow in grasping the full significance of molecular weights and gram- 

 molecular volume. Since we arrive at approximate atomic weights 

 through molecular weights, in the case of most elements at least, and 

 since the application of the law of combining volumes necessitates a 

 real familiarity with the volume of the gram-molecule, it is important 

 that students become familiar with molecular weights, not merely as 

 they are represented by a formula so readily written by the instructor, 

 but by actual determination of the molecular weights of the more com- 

 mon elements or compounds as they are presented in the class or lecture 

 room. 



The ordinary method used is to evacuate a small glass balloon 

 and then fill it with the gas under consideration. From the weight of 

 the empty balloon, the weight filled with the gas, and the capacity 

 of the balloon, the density of the gas compared with hydrogen is ob- 

 tained and this value doubled gives the molecular weight. 



A balloon flask cannot be used conveniently as indicated unless an 

 automatic generator is being used for producing the gas, or unless 

 the gas is stored under very moderate pressure, as in an ordinary 

 gasometer. 



The author has found a satisfactory solution of the problem by 

 employing a light weight gas pipette having a capacity of approxi- 

 mately 500 cc and fitted at each end with a glass stopcock. For more 

 accurate weighing a similar pipette but without the stopcocks is used 

 as a tare. A lecture balance weighing to a centigram or a little less 

 will give results sufficiently accurate for the purpose. 



Previous to any determinations of molecular weights in class, the 

 instructor will have determined the capacity of the pipette and its. 

 weight when evacuated. In the lecture room the pipette is simply 

 introduced into the stream of gas being prepared for illustrative pur- 

 poses. Hydrogen, oxygen, chlorine, hydrogen chloride, nitrogen, car- 

 bon dioxide, etc., fills the pipette as it flows from the generating vessel 

 on its way to the pneumatic trough or receiving jars, the gas being 

 dried if a little greater accuracy is desired. The pipette is supported 

 in an upright position by a suitable clamp and the gas is introduced 

 at the top when lighter than air, at the bottom when heavier. By the 

 time the instructor has collected all the gas necessary for his lecture 

 demonstrations, the air in the pipette will have been completely re- 

 placed by the gas under investigation. The stopcocks are closed, the 

 pipette removed from the system and weighed. Only one weighing being 

 necessary, little time is consumed for the entire operation. From a 

 wall chart prepared for the purpose, the factor for reducing volume to 



"Proc. Ind. Acad. Sci.. vol. 33, 1923 (1924)." 



