Question

What is the equilibrium constant of pure water at 25°C?

Answer 1

`K_w = 1.0xx10^(-14)` at `25ºC`.

Explanation:

Sometimes, in pure water, one proton (`H^+` ion) of one water molecule will be attracted to one of the lone pairs in another water molecule.
This causes the `H^+` ion to leave its current water molecule (leaving behind `OH^-`) to attach to the lone pair of another water molecule (forming `H_3O^+`, the hydronium ion).

This process is called the auto-ionization or self-ionization of water, since `H_2O` is ionizing into `OH^-` and `H^+`.

So, this reaction will sometimes occur:

`H_2O + H_2O rightleftharpoons H_3O^+ + OH^-`

However, it's important to note that this reaction only happens very rarely. As such, the equilibrium constant, which is a measure of the concentration of products to the concentration of reactants, will be very low.

In fact, at `25ºC`, this equilibrium constant will be `1.0xx10^(-14)`.