Water Ion Product
Self-ionization constant of water at 25°C.
Formula
Variables
Example
Constant in aqueous solutions
How it works
Pure water does not stay entirely as H2O molecules; a tiny fraction dissociates into hydrogen ions and hydroxide ions. At 25°C, the equilibrium between these ions has a fixed product, Kw = [H+][OH−] = 1.0 × 10^−14, because raising one ion concentration lowers the other in a way dictated by the water autoionization equilibrium. This constant is the basis for calculating pH and pOH in aqueous solutions and for relating acidity to basicity.
When to use it
Use this when you need to find pH, pOH, [H+], or [OH−] for water-based solutions at 25°C, especially in acid–base equilibrium problems. It is also useful when checking whether a solution is acidic, neutral, or basic from one ion concentration.
Worked example, step by step
Find the hydroxide ion concentration in a solution at 25°C with [H+] = 3.2 × 10^−6 M. Use the water ion product: Kw = [H+][OH−] = 1.0 × 10^−14 Rearrange for hydroxide concentration: [OH−] = Kw / [H+] Substitute the values: [OH−] = (1.0 × 10^−14) / (3.2 × 10^−6) Divide the coefficients and subtract exponents: [OH−] = 0.3125 × 10^−8 M = 3.1 × 10^−9 M Final answer: [OH−] = 3.1 × 10^−9 M
Common mistakes to avoid
- Using Kw = 10^−14 at temperatures other than 25°C without checking the temperature dependence
- Confusing [H+] and pH, or [OH−] and pOH, instead of using concentration units directly
- Forgetting to rearrange the equation correctly, especially when solving for the ion concentration that is not given
Did You Know?
Even pure water faintly self-ionises, and that constant, Kw, is what fixes neutral pH at exactly 7.
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