CRITICAL VOLUME 35 



Volume difference for CH 2 

 56-5 

 55-8 



57-4 



Now making use of the critical volume of normal pentane 

 C 5 H 12 , which is 307*2, we get 



H 2 = C 5 H ]2 -5CH 2 = 307-2-5x56-6 = 24-2, 

 and C - 



which then leads to the value 29 for oxygen, not very 

 different from that stated above. 



Here it must be remembered that where the critical 

 data show abnormalities, as for acetic acid, apparently 011 

 account of the formation of double molecules, the above 

 relation also ceases to be true. The molecular volume of 

 acetic acid at the critical point is 



60 



i. e. far removed from that of the isomeric methyl formate 

 171. 



3. Volume at the Soiling Point. 



Since the molecular volumes at the critical point show 

 additive properties, so also do the volumes at corresponding 

 temperatures, since these are equal fractions of the critical 

 temperatures, and therefore correspond to equal fractions 

 of the critical volumes. Thus the additivity existing in 

 the critical state leads to that discovered by Kopp for 

 molecular volumes at the boiling point, for the boiling 

 points are on account of the approximate equality of 



C 2 



