LAWS OF ELECTROLYTIC DISSOCIATION 41 



demands that the sum of the total positive ions be equal to the sum 

 of the total negative ions: 



[H+] = [S-] + [0H-] (2) 



and thirdly, 



IOH-, = j^ (3) 



By eliminating [S~] in equation (2) by using (1), and then sub- 

 stituting for [0H~] by the use of (3), we obtain 



w 



^ kjA] k^ 

 [H+] "^ [H+] 



or 



[H+] = Vk [A] + kw (4) 



It is to be observed from the above that the [H+] may never 

 become less than k^, or below true neutrality, not even with an 

 infinitely small k; and also that acids whose k differs very little 

 from kw raise the [H+] of water to so slight an extent that solutions 

 of sugar, for instance, are to be considered as being virtually 

 "neutral." The acid nature of sugar becomes evident only in its 

 ability to raise the [H+] of solutions of bases, in which case the 

 [0H~] concentration is decreased without the sugar actually "bind- 

 ing" the NaOH molecule. 



In a mixture of two weak acids, whose total concentrations are 

 Ai and A2, the concentrations of undissociated molecules are Si 

 and S2 and the dissociation constants are ki and k2 respectively, the 

 [H+] may be calculated in the following way: 



1. [Sr] + [S2-] = [H+] 



[S2-] lH+1 , 



Hence 



[H+] = VkiSi + k2S2 



