In chemistry, Kw is the ion product constant for water, which is the equilibrium constant for the dissociation reaction of waters ions/Text/12%3A_Acids_and_Bases/12.5%3A_pH_and_Kw). It is written as Kw instead of Keq, and its value is 1.0 x 10^-14 at 25°C/Text/12%3A_Acids_and_Bases/12.5%3A_pH_and_Kw). Kw is a useful tool that combines conceptual knowledge from branches like equilibrium, acids/bases, and properties of water. The concentration of H3O+ and OH- are equal in pure water because of the 1:1 ratio of the dissociation reaction, which means that /Text/12%3A_Acids_and_Bases/12.5%3A_pH_and_Kw). The pH and pOH of a solution can be calculated using Kw, and pH + pOH = 14/Text/12%3A_Acids_and_Bases/12.5%3A_pH_and_Kw).