Ion Product of Water Calculator Formula

Understand the math behind the ion product of water calculator. Each variable explained with a worked example.

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Formulas Used

Kw

kw = conc_h * conc_oh

pKw

pkw = -log10(conc_h * conc_oh)

Variables

VariableDescriptionDefault
conc_hHydrogen Ion Concentration [H⁺](M)1e-7
conc_ohHydroxide Ion Concentration [OH⁻](M)1e-7

How It Works

Ion Product of Water

Formula

Kw = [H⁺] × [OH⁻]

At 25°C, Kw = 1.0 × 10⁻¹⁴, meaning:

  • In pure water, [H⁺] = [OH⁻] = 1.0 × 10⁻⁷ M
  • If [H⁺] increases, [OH⁻] decreases proportionally
  • pKw = pH + pOH = 14 at 25°C

    • Worked Example

    Pure water at 25°C has [H⁺] = 1.0 × 10⁻⁷ M and [OH⁻] = 1.0 × 10⁻⁷ M.

    conc_h = 1e-7conc_oh = 1e-7
    1. 01Kw = [H⁺] × [OH⁻]
    2. 02Kw = (1.0 × 10⁻⁷)(1.0 × 10⁻⁷)
    3. 03Kw = 1.0 × 10⁻¹⁴
    4. 04pKw = 14.0

    Frequently Asked Questions

    What is the ion product of water?

    Kw is the equilibrium constant for the autoionization of water: H₂O ⇌ H⁺ + OH⁻. At 25°C it equals 1.0 × 10⁻¹⁴.

    Does Kw depend on temperature?

    Yes. Kw increases with temperature because autoionization is endothermic. At 60°C, Kw is about 9.6 × 10⁻¹⁴.

    Is water neutral at all temperatures?

    Pure water is always neutral ([H⁺] = [OH⁻]), but pH of neutral water is only 7.0 at 25°C. At higher temperatures, neutral pH is less than 7.

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