What is the concentration of hydronium ions in pure water?

The ph scale is based on the self-ionisation of pure water. Two water molecules will sometimes combine into hydronium and hydroxide ions. Pure water is considered to neutral and the hydronium ion concentration which is equal to the hydroxide ion concentration.

In an aqueous solution, the hydronium ion concentration is equivalent to the hydrogen ion concentration because the hydrogen ion exists as the hydronium ion. The pH of pure water is 7, and:

pH = - log[H+]

7 = - log[H+]

[H+] = 1 x 10⁻⁷

Therefore, the hydronium ion concentration in pure water is also 1 x 10⁻⁷.