Ionic Product of Water ([H⁺][OH⁻]) Calculator
Calculate [H⁺] or [OH⁻] from Kw = [H⁺][OH⁻].
Formula first
Overview
The ionic product of water, Kw, is the equilibrium constant for the self-ionization of water into hydronium and hydroxide ions. At a standard temperature of 25°C, this constant remains fixed at 1.0 × 10⁻¹⁴, defining the inverse relationship between acidity and alkalinity in aqueous solutions.
Symbols
Variables
[H^+] = [H+], [OH^-] = [OH-], K_w = Kw Constant
Apply it well
When To Use
When to use: Use this equation whenever you need to convert between hydrogen ion concentration and hydroxide ion concentration in an aqueous environment. It is valid for acidic, basic, and neutral solutions, provided the system has reached chemical equilibrium.
Why it matters: This equation forms the mathematical basis for the pH scale, which is essential for monitoring environmental water quality and human physiology. It ensures that any change in acidity is balanced by a corresponding change in basicity, maintaining chemical stability in biological cells.
Avoid these traps
Common Mistakes
- Forgetting Kw changes with temperature.
- Using wrong value for Kw.
One free problem
Practice Problem
A sample of pure water at 25°C has a hydrogen ion concentration of 1.0 × 10⁻⁷ M and a hydroxide ion concentration of 1.0 × 10⁻⁷ M. Calculate the value of the ionic product of water (Kw).
Solve for:
Hint: Multiply the concentration of the hydrogen ions by the concentration of the hydroxide ions.
The full worked solution stays in the interactive walkthrough.
References
Sources
- Atkins' Physical Chemistry
- IUPAC Gold Book: Ionic product of water
- Wikipedia: Ionic product of water
- IUPAC Gold Book
- NIST Chemistry WebBook
- IUPAC Gold Book: 'autoprotolysis constant of water'
- NIST Chemistry WebBook: 'Water (H2O) - Ionization Constant'
- Atkins, P. W., & de Paula, J. (2018). Atkins' Physical Chemistry (11th ed.). Oxford University Press.