88 HYDROGEN ION CONCENTRATION 



slightly acid reaction. Let us now study the degree of hydrolysis 

 7 of the salt which is defined as 



■ ■ = [1!1 



^ [C] 



where [C] denotes the total concentration of the salt. 



For the general solution of the problem let us consider the co- 

 existence of the following laws: 



[H+] X [0H-] = k^ (1) 



which is the dissociation equation of water. 



Since the acid produced in the hydrolysis was assumed to be a 

 strong acid, its concentration is therefore = [H+]. But since in 

 the hydrolysis as much base is produced as acid, it follows that 



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



Furthermore the equation for the dissociation of the base is 



[BOH] ^^'^ ^^^ 



and finally, 



[B+] + [BOH] = [C] (4) 



From these four equations involving the four unknown values 

 [H+], [0H-], [B+] and [BOH] each of the latter may be calculated, 

 and knowing the value of [H+] that of 7 may be obtained. But 

 these calculations are of a rather involved character. It suffices 

 ordinarily to carry out the calculations, with some simplifying 

 assumptions, for certain limits, as: 



a. Assuming that the hydrolysis is very slight (7 < 0.01, [H+] 

 < 10-3). 



Then in equation (4) the value of [BOH] as a term of a sum is 

 negligible in respect to that of [B+] and in this case 



[B+] = [C] (4a) 



and solving for [H+] by substituting in (2) the value for [OH"] from 



