The dissociation and ionic product of water

Kohlrausch and Heidweiller (1894) found, after careful experimental studies, that the purest water possesses a small, but definite conductance. Water must therefore be slightly ionized in accordance with the dissociation equilibrium  

Applying the law of mass action to this dissociation, we can express the equilibrium constant as 

From the experimental values obtained for the conductance of water the value of K can be determined this was found to be 1 -82 x 10 16 at 25°C. This low value indicates that the degree of dissociation is negligible all the water can therefore in practice be regarded as undissociated. Thus the concentration of water (relative molecular mass = 18) is constant, and can be expressed as 

The importance of the ionic product of water lies in the fact that its value can be regarded as constant not only in pure water, but also in diluted aqueous solutions, such as occur in the course of qualitative inorganic analysis. This means that if, for example, an acid is dissolved in water, (which, when dissociating, produces hydrogen ions), the concentration of hydrogen ions can increase only at the expense of hydroxyl-ion concentration. If, on the other hand, a base is dissolved, the hydroxyl-ion concentration increases and hydrogen-ion concentration decreases. 

We can define the term neutral solution more precisely along these lines. A solution is neutral if it contains equal concentrations of hydrogen and hydroxyl ions that is if 

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