In a neutral solution, what is the concentration of hydronium ions?

In a neutral solution at 25°C, the concentration of hydronium ions (H₃O⁺) is defined to be 10⁻⁷ mol L⁻¹. This is because pure water dissociates to produce an equal number of hydronium and hydroxide ions, leading to a balance where both concentrations are equal in neutral conditions.

Since the water dissociation constant (Kw) is 1.0 x 10⁻¹⁴ at this temperature, the relationship between the concentrations of hydrogen ions and hydroxide ions is represented by the equation:

[ K_w = [H_3O^+][OH^-] ]

In neutrality, where [H₃O⁺] = [OH⁻], each concentration must therefore be the square root of Kw, leading to:

[ [H_3O^+] = \sqrt{K_w} = \sqrt{1.0 \times 10^{-14}} = 1.0 \times 10^{-7} , \text{mol L}^{-1} ]

Thus, in a neutral solution, the concentration of hydronium ions is indeed 10⁻⁷ mol L⁻¹