If the pH changes the density

to directory mode

Ion product

If we apply the law of mass action to the autoprotolysis equilibrium, we get

Density of water at 25: 996 /:

But because the total concentration of H2O practically does not change, if only some molecules undergo autoprotolysis, the total concentration of H is2O almost equal to the H2O concentration in equilibrium. Then [H2O]2 can be included in the constant, and one obtains:

The equilibrium constant is called the ionic product of water. Its value depends on the temperature; at 25 = 1⋅ 10−14 .

Tab. 1
Ion product of water temperature dependence
T [] []

The value of increasing with temperature corresponds to the endothermic process of autoprotolysis or, conversely, to the exothermic neutralization reaction.

Since oxonium and hydroxide ions are formed to the same extent during autoprotolysis, the following applies to pure water at 25:

The ion product is an equilibrium constant not only for pure water, but also for all dilute aqueous solutions, i.e. it also applies to dilute acids and bases in which there is a high excess of oxonium or hydroxide ions. If it is to remain constant, the addition of H3O+ the concentration of OH decrease accordingly.

If you consider hydrochloric acid with a concentration of 0.01, then, because HCl is completely converted in water:

If you insert this concentration into the relationship for the ion product of water, the result is the concentration of OH:

Acidic aqueous solutions therefore have an excess of oxonium compared to hydroxide ions, while basic solutions have an excess of hydroxide ions. With the value for [H3O+] is via the ion product also the value for [OH] given and vice versa. It is therefore sufficient to specify a concentration.