For any solution at 25oC, the product of the [OH−] and [H3O+] always equals a defined constant:
Kw=[OH−][H3O+]=1.00×10−14M2
The constant Kw is the equilibrium constant for water, called the ion-product constant, and is equal to 1.00×10−14M2
From this relationship, we can see that [OH−] and [H3O+] are inversely proportional; as the concentration of hydroxide ions increases, the concentration of hydronium/hydrogen ions decreases.
An acidic solution is one in which the hydronium ion concentration is greater than that of the hydroxide ion concentration. Why? Because the acidity of a solution can be measured by the solution's pH; a lower pH indicates a higher [H3O+]. If the pH<7, the solution is said to be acidic, and [OH−]<[H3O+]. For a neutral solution, pH=7, and the hydronium and hydroxide concentrations are equal. If the pH is higher than 7, the solution is basic, and because of the equation
Kw=[OH−][H3O+]=1.00×10−14M2
the [OH−] is larger than the [H3O+]. Thus, if [OH−]>[H3O+], the solution is basic (pH>7).
