6 HYDROGEN ION CONCENTRATION 



or abbreviated, the reaction 



2 mol, H2 + 1 mol. O2 ;::i 2 mol. H2O 

 is in equilibrium when 



[H2J2 ■ [O2] 



[H20]2 



= k 



This law of mass action holds good only under the assumption 

 that all of the interreacting molecular complexes as well as those 

 newly formed coexist in a homogeneous solution (or in gaseous 

 state). If one of the substances involved in a reaction is partially 

 precipitated (removed from solution) because of oversaturation, 

 then the term "concentration" of this substance should comprise 

 only the concentration of its part still remaining in solution. 



Should approximate accuracy be accepted as satisfactory, then 

 it could be stated that the law of mass action is confirmed by so 

 large a number of cases that it is to be regarded as a general law 

 also valid for those cases in which the theoretical proof of its cor- 

 rectness could be not obtained. But in attempt at a higher degree 

 of accuracy it must be recognized that the law in the form stated 

 above is only approximately correct. The smaller the concentra- 

 tion of the reacting substances the closer the approximation, and 

 it is closest for ideal gases and for substances reacting in extremely 

 dilute solutions. In fact, the strict thermodynamic test permits 

 the application of the law to these latter extreme cases only. This 

 test leads to a really general law which differs from the above 

 stated law insofar as the term ''concentration" is everywhere re- 

 placed by the term "pressure." It may either be gas pressure or 

 osmotic pressure. Since, in the state of very great dilution, pres- 

 sure and concentration are proportional to each other, it follows 

 that under this condition one may simply substitute concentration 

 for pressure. The thermodynamic form of the law is vahd for any 

 order of magnitude of pressure. But at high pressures concentra- 

 tion and pressure are no longer proportional. Hence the theo- 

 retical explanation of the fact that at the higher concentrations the 

 law of mass action, expressed in terms of concentration, cannot be 

 absolutely correct. In order to render it in an absolutely valid form 

 it would be necessary to multiply every factor involving concen- 

 tration by an appropriately chosen factor. This factor is generally 

 less than 1. The values so corrected are called active masses. 



