How is the concentration of hydronium related to hydroxide at equilibrium?

The relationship between the concentration of hydronium ions (H₃O⁺) and hydroxide ions (OH⁻) at equilibrium in aqueous solutions is described by the ion product constant for water, which is defined as:

Kw = [H₃O⁺][OH⁻] = 1 x 10⁻¹⁴ at 25°C.

This relationship indicates that the product of the concentrations of hydronium and hydroxide ions in a neutral solution is always equal to 1 x 10⁻¹⁴ at this temperature. Therefore, if you know the concentration of one ion, you can determine the concentration of the other ion based on this constant.

For example, in pure water at 25°C, the concentration of hydronium ions equals the concentration of hydroxide ions, both being 1 x 10⁻⁷ M. When the concentration of one increases (meaning the solution becomes more acidic), the concentration of the other must decrease accordingly, maintaining the constant of 1 x 10⁻¹⁴.

This fundamental principle is crucial in understanding acid-base chemistry and the behavior of aqueous solutions in equilibrium.