LAWS OF ELECTROLYTIC DISSOCIATION 29 



9. The general application of the mass law to the electrolytic 



dissociation of acids 



It was shown above how electrically neutral molecules like those 

 of water can break up into two oppositely charged ions. The 

 reverse phenomenon by means of which these two ions can combine 

 to form electrically neutral molecules can be shown just as well. 

 It will depend upon the external conditions whether in any given 

 case the reaction will proceed in one or the other direction. A 

 free H-ion and a free OH-ion coming spatially into near proximity 

 of each other as a result of molecular motion will unite to form H2O. 

 On the other hand the impact of a heterogeneous molecule, occur- 

 ring in a suitable way, will split off from the H2O molecule a H- 

 ion. In general, for every given external condition (especially that 

 of temperature) there will be established a certain statistically con- 

 sidered situation in which the number of ions forming is equal to 

 that of the disappearing ions. But such are exactly the conditions 

 for the application of the law of mass action, and the same symbols 

 as are used for stating the reactions of non-electrolytes may be 

 employed in this case. Thus to the "chemical equation," 1 mol 

 alcohol + 1 mol acetic acid ;=i 1 mol ethyl acetate + 1 mol H2O, 

 corresponds the equation: 



1H+ + 1 OH- = 1 H2O 

 And the formula of the mass law as used in 



[EtOH]X[AcOH] 

 [EtOAc] X [H2O] 



corresponds in this case to 

 [H+] X [0H-] 



= k 



[H2OI 



= k, or, [H+] X [0H-] = kw. 



This states the law of the electrolytic dissociation of water which 

 has been discussed earlier. The dissociation of an acid dissolved 

 in water will now be considered. If the acid is designated as HS 

 and its component ions as H+ and S~, then the mass law can be 

 stated in the form of the following equilibrium equation : 



[HS] ^ ^^> 



