220 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



In the mixture, therefore, assuming that no displacement of the equi- 

 librium takes place, we should have for the concentration of the ions M x + 



the value 1+ * , for that of the common ion * * J" v 2 - and for 



V, + V 2 Y! -j- V 2 



y 

 that of the un-ionized fraction M^,, * y . If the law of mass-action 



1 I 2 



holds, we have the equation: 



If M! + = M 2 + , the expression for the concentration of the common ion 

 becomes : 



and the equilibrium equation reduces to: 



M + x x x - _ 



In other words, if the concentration' of the common ion is the same in the 

 original solutions, then, if these solutions are mixed in any proportion, 

 assuming no change in the equilibrium to take place, the concentrations 

 of the ions in the mixture will be such as to fulfill the conditions necessary 

 for equilibrium. 



The correctness of this principle may readily be tested in the case of 

 weak acids. Since the conductance in solutions of the acids is due chiefly 

 to the conductance of the hydrogen ion, it follows that two acids will 

 have the same concentration of the hydrogen ion when the solutions 

 have the same specific conductance. Therefore, a mixture of two solu- 

 tions fulfilling these conditions will have the same specific conductance 

 as the original solutions. If the anions have different conductance 

 values, the specific conductance of isohydric solutions will differ in 

 proportion to the conductance of these anions, and the specific conduct- 

 ance of a mixture of the solutions will be the arithmetic mean of that 

 of the components. This principle has been extensively tested by the 

 conductance as well as other methods and has been shown to hold true 

 for mixtures of weak acids and bases. 



It has been found, however, that even in solutions which do not con- 

 form to the law of mass-action, that is, in solutions of strong electrolytes, 



