By Timothy Banas, Updated Aug 30, 2022
In pure water, a small fraction of molecules undergo auto‑ionization, generating hydronium (H₃O⁺) and hydroxide (OH⁻) ions. The concentration of H₃O⁺ determines a solution’s acidity, quantified by its pH. The pH scale ranges from 0 (highly acidic) to 14 (highly alkaline), and it is a logarithmic measure of the hydronium ion concentration.
Identify the pH of the solution. For known reagents, the value is usually printed on the label or listed in a reference table. If the pH is unknown, measure it with a calibrated pH meter or perform a titration.
The relationship between pH and hydronium concentration is:
pH = –log₁₀[H₃O⁺]
Rearranging gives:
[H₃O⁺] = 10–pH
Insert the measured pH value into the equation. For example, a solution with pH = 2 yields:
[H₃O⁺] = 10–2 = 1.0 × 10⁻² mol L⁻¹
Thus, the solution contains 0.01 moles of hydronium ions per liter.
