pH calculator
Use the dilute aqueous solution approximation with an equilibrium ion concentration.
Formula
pH ≈ −log₁₀[H₃O⁺]; pOH ≈ −log₁₀[OH⁻]; pH + pOH ≈ 14 at 25 °C (mol/L)
Equilibrium ion concentration: 0.001 mol/L · pH ≈ -log10((0.001 mol/L × 1,000 mol/m³/mol/L)÷1000) · pOH ≈ 14-(3)
How to use
- Choose hydronium or hydroxide.
- Enter its positive equilibrium concentration.
- Read pH and pOH for 25 °C.
Examples
- [H₃O⁺] = 0.001 mol/L gives pH 3 and pOH 11.
Frequently asked questions
Can I enter the initial acid concentration?
Only if it equals the equilibrium ion concentration; weak acids need an equilibrium calculation.
Is this the exact definition of pH?
No. pH is defined using ion activity. Concentration approximates it in dilute solutions.
Does the sum always equal 14?
This tool assumes 25 °C. Water’s ionization constant changes with temperature.
Sources
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