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Practical class 27.

Theme: Application of the law of mass action to the process of dissociation of water. pH scale.

Chemically pure water conducts an electric current very poorly and has an electrical conductivity of 0.055 µS∙cm⁻¹. But nevertheless it has a measurable electrical conductivity that is explained by the slight dissociation of water into hydrogen and hydroxide ions (According to the theories of Svante Arrhenius):

H₂O → H⁺ + OH^–

The electrical conductivity of pure water can be used to calculate the concentration of hydrogen and hydroxide ions in water. Let us write an expression for the dissociation constant of water:

We can rewrite this equation as follows:

[H⁺]∙[OH^–] = [H₂O]∙K

Replacing the product [H₂O]∙K in the last equation with the new constant K_w, we have:

[H⁺]∙[OH^–] = K_w

The latter K_w is called the ion product of water (or ionization constant, dissociation constant, self-ionization constant). The K_w value is depended of temperature. For pure water at 25°C K_w=10^-14, we have

[H⁺] = [OH^-] = 1·10^-7 mol/L.

Hence, for this temperature:

K_w = 10^-7 ∙ 10^-7 = 10^-14

Solutions in which the concentrations of the hydrogen ions and hydroxide ions are the same are called neutral solutions.