Section /j. Hydrolysis of Ammonium Acetate. 185 



73. THE HYDROLYSIS OF AMMONIUM ACETATE AND THE IONIZATION- 



CONSTANT OF WATER. 



From table 67 it will be seen that at 18 the excess of acid or base 

 causes an increase of only 0.2 to 0.5 per cent in the conductance, in cor- 

 respondence with the small degree of hydrolysis known to exist at this 

 temperature. An accurate calculation of it from these results is therefore 

 not possible. At the higher temperatures, however, the increase is consid- 

 erable about 5 per cent at 100, and 15 to 21 per cent at 156. At 100 

 the addition of a quantity of acid or base equivalent to the salt produced 

 as- great an increase as a larger quantity, showing that the hydrolysis had 

 been reduced substantially to zero. The effect of the acid was, as it 

 should be, nearly equal to that of the base, the small differences observed 

 being doubtless due to experimental error. The percentage increase was 

 also nearly the same at the two concentrations of the salt (4.6 and 4.9 

 per cent) respectively, showing that the hydrolysis does not increase much 

 with the dilution, which is what the mass-action law requires for a salt 

 whose acid and base are both weak. At 156 the second equivalent of acid 

 or base produces a large further increase in conductance, showing that 

 the salt is still somewhat hydrolyzed. Here again the acid and base have 

 not far from the same effect, as they should have on account of the small- 

 ness of their ionization constants. 



The quantitative calculation of the ionization at 100 is comparatively 



simple. Since the hydrolysis is reduced to zero by the added acid or base, 



the increase in specific conductance produced by it when divided by the 



equivalent conductance A (338) of the completely ionized salt gives at 



once the number of equivalents per cubic centimeter of free acid and base 



which have been converted into ions. In addition a quantity of the 



un-ionized salt, corresponding to the increased concentration of its ions, is 



produced out of the acid and base. To compute this, we have made use 



(Cy) n 



of the equation ^, r-r- = K (where y is the fraction ionized and 



^ C(l y h) 



h the fraction hydrolyzed), in which we have determined the constants n 

 and K from the conductances (lba) of the unhydrolyzed salt (7,141 and 

 2,990 X 10' G ) at the two concentrations (24.92 and 9.97 milli-equivalents 

 per liter) investigated and from the A value for the salt.* We have then 

 calculated from the values of l/A , which are equal to Cy, the concentra- 

 tion of un-ionized salt, C(l y h), both in the solution containing the 

 salt alone and in that to which acid or base had been added. 



*For this last calculation we used a preliminary value of A, namely 333 instead 

 of 338; but this could have only an inappreciable influence on the result. The 

 numerical equation so obtained when the concentration is expressed in milli-equiva- 

 lents per liter is: logioC(l y h) = 1.443 log 10 (Cy) 1.379. 



