50 THE TRUE VALUE OF a OF VAN DER WAALS' 



being trivalent so that the number of. valences in the molecule is 

 16, is 30.47 X 10 12 , a rigure in very good agreement with the others 

 found. Van Laar gives a value corresponding to 27.19 X 10 12 

 which is again, as so often, too low. 



36. Brom-benzene. C Q H b Br. 



(«). a = 2.03 BT e V c / 1 .03 — P,.V 2 . T r = 670 ; P, = 33.9 1 2 



mm. Eg. ; d c = . 4853 ; V,. = 323.4 .... a = 30.80 X 10 12 



(4). « = 6.5P/ C 2 a = 31.46 X 10' 2 



( c ). a = ix'M 2 /Sd t - l: \ /a = 55.37 M = 1 56.96 a = 30.81 X 10 2 



(i). a = CRT,/:. C' = 1 .69 I a = 30.48 X 10 12 



(e). a = 3 M 7V 1/3 7;. C/<. This calculation was 

 made from the uncorrected result of 

 Morgan and Daghjjan by the drop 

 weight method. At 59.7° sv 2 ^ was found 

 to be 704.4 ergs, d was 1.4438. D was 

 neglected. T c — T was 337.3. This gives a = 30.72 X N> 12 



The mean of these various values is 30.77 X 10 12 . The value requi- 

 red by the computation from the molecular weight and the valences, 

 taking bromine as monovalent, so that there are 30 valences in 

 the molecule, is 33.05 X 10 12 . ^he value 30.77 would correspond 

 to only 27 valences in the molecule. This appears then to be a 

 real exception to the rule correlating cohesion with gravitation and 

 valence number. It may be, however, that it is only an apparent 

 exception. The critical data of brom-benzene had to be determined 

 by extrapolation over a long distance. They are not, therefore, very 

 reliable. In the case of the surface tension we have to use the 

 uncorrected measurements. The effect of the Loiinstein correction 

 is generally to raise the surface tension measurement, but it is not 

 likely to raise it 7—8 °/ . The value 31.46 found from the 6.5 P,,V, 2 

 formula is only between 3 — 4 / too low. It is quite possible that 

 correction of the critical data would bring it into line. Van Laar's 

 value for a 1/2 is .242, which gives fora the value 25.70 X 10 12 . 

 This as will be seen is very much below the mean value of 30.72. 



37. n-llepiane. 6' 7 i/ 16 . 



[a), a = 2.03 B2 , e F c /l.0B — P c F c 2 . T v = 539.85; P,.= 20.430 

 mm.Hg.; ;/ c = .2341; F c = 427.17.. . a = 32.84X»0 12 



(b). a = 6.5 P,.V, 2 a = 32.30 X I0 12 



(c). a = fj!M 2 l?> < 2/:! . fi' = 98.64; M = 100 . a = 30.23 X 10' 2 



{d). a = C'BT C F C . C' = 1.8 14 a = 34.80 X N>' 2 



(e). Could not find surface tension of heptane. 



