Dissociation of Water Vapour. 

 Table II. 



721 



Temp. abs. 



Percentage decomposition 

 at 760 mm. pressure. 



K. 



1233° 



0001087 



3-768 



1273 



0001864 



3-783 



1413 



0-008366 



3-768 



1413 



0-00906 



3-803 



1464 



001658 



3-857 



1487 



0-01612 



3753 



1598 



0-04992 



3-849 



1598 



0-04963 



3-844 



1605 



0-04719 



3-802 



1611 



0-04996 



3-806 



1614 



0-05263 



3-818 



1625 



0-05855 



3-830 



1628 



005716 



3-809 



1628 



0-05628 



3-803 



1633 



005825 



3-805 



1636 



0-0591 



3-797 



1637 



006118 



3-810 

 Mean =3-806 



The third column in these tables contains the values for 

 the equilibrium constant calculated from the percentage 

 decomposition. 



Three formula? have been derived thermodynamically by 

 Nernst and Wartenburg for calculating this constant K. 

 The first was published in the Gottingen Nachrichten in 1905, 

 and in a somewhat simplified form was used by Langmuir: — 



l0g (2 + x)(l-xj 



1U1 25030 , ogK1 T 

 = 11-51 y~ + 2 ' 651o 8iooo 



0-00055(T-1000); 



neglect x compared to 1, and divide by 3, 



log x = o'So 



°^P + 0-88 log I ^- 0-00018(T -1000). 



T is the absolute temperature, x the percentage decom- 

 position. The constant K has therefore the value 3*83. At 

 high temperatures it is necessary to substitute for x the 

 value 



x 





a/(^1> 



i-*) 3 



The experimental value for K determined by Langmuir 

 from this formula is 3*79, which represents a difference of 

 about 7 per cent, in the amount of decomposition from that 



Phil. Mag. S. 6 Vol 17 No. 101. May 1909. 3 C 



