Ion-product constant for water, Kw

Definition

The ion-product constant for water, $K_w$, is the equilibrium constant for the self-ionization of water. It is defined as the product of the molar concentrations of hydrogen ions and hydroxide ions in water at a given temperature.

5 Must Know Facts For Your Next Test

1. $K_w$ at 25°C is $1.0 \times 10^{-14}$.
2. The equation for $K_w$ is $[H^+] [OH^-] = K_w$.
3. $K_w$ changes with temperature; it increases as temperature increases.
4. In pure water at 25°C, $[H^+] = [OH^-] = 1.0 \times 10^{-7}$ M.
5. $K_w$ is fundamental to understanding pH and pOH calculations.

Review Questions

Related terms

pH:

$\text{pH} = -\log[H^+]$, a measure of acidity or basicity of an aqueous solution.

pOH:

$\text{pOH} = -\log[OH^-]$, a measure similar to pH but focuses on hydroxide ion concentration.

Self-Ionization of Water: A process where water molecules dissociate into hydrogen ions ($H^+$) and hydroxide ions ($OH^-$).