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CHEMISTRY · CALCULATE · UNDERSTAND

pH Calculator

Calculate pH, pOH, free hydronium, and free hydroxide concentrations in aqueous solutions. Default pKw = 14 is the common 25 °C value.

General chemistry

Enter your values

Use a decimal point or scientific notation (1e-3). Choose units before calculating.

Choose which supplied value to use. The other input fields are hidden; unit selectors remain available for reporting concentrations.
Enter a positive free-ion concentration, not the formal molarity of a weak acid.
Input unit when [H₃O⁺] is known; otherwise controls its reported result unit.
Input unit when [OH⁻] is known; otherwise controls its reported result unit.
Leave blank to use 14.00, the common value for water at 25 °C. Supply pKw for another temperature or specified medium.
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Your result

Enter your values and press Calculate to see your answer and all five solution steps.

pH = −log₁₀[H₃O⁺]; pOH = −log₁₀[OH⁻]; pH + pOH = pKw

Common Mistakes

  • Do not enter the analytical molarity of a weak acid or weak base as [H₃O⁺] or [OH⁻]; first determine the equilibrium free-ion concentration.
  • Do not assume pH + pOH = 14 at every temperature; use the pKw appropriate to the stated conditions.
  • Do not confuse mmol/L with mol/L: 1 mmol/L = 1 × 10⁻³ mol/L.

Assumptions and limitations

  • At 25 °C in ideal dilute water, pKw ≈ 14 and neutral water has pH = pOH = 7. The neutral point is pKw/2 when a different pKw is supplied.
  • Supported scope: direct aqueous free [H₃O⁺]/[OH⁻] concentration–pH/pOH conversions. It does not solve acid dissociation, base dissociation, buffers, titrations, or activity-coefficient models.

pH = −log₁₀[H₃O⁺]; pOH = −log₁₀[OH⁻]; pH + pOH = pKw

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