Introduction to pKw
H2O can act as both an acid and a base, and the proton transfer reaction shown below occurs to a very small extent in pure water.
The equilibrium constant for the reaction is referred to as Kw and has the value shown below at 25 °C.
The magnitude of Kw at all temperatures is small, and it is convenient to use pKw which is defined similarly to pH.
pOH can be analogously defined (-log [OH–]).
The relationship below follows from the rules for combining exponents that are given.
If any two of these variables in this relationship are known, the third can be calculated.
The equilibrium constant for the reaction is referred to as Kw and has the value shown below at 25 °C.
H2O(l) + H-OH(l) 
H3O+(aq) + –OH(aq)
Kw = [H3O+][OH–]
At 25 °C Kw = 10–14
H3O+(aq) + –OH(aq)Kw = [H3O+][OH–]
At 25 °C Kw = 10–14
pKw = -log Kw
pKw = 14 at 25 °C
Like all equilibrium constants, the magnitude of Kw depends on temperature. pKw = 14 at 25 °C
The magnitude of Kw at all temperatures is small, and it is convenient to use pKw which is defined similarly to pH.
| Useful maths: |
| 10a × 10b = 10a+b |
| 10a ÷ 10b = 10a-b |
The relationship below follows from the rules for combining exponents that are given.
pKw = pH + pOH
If any two of these variables in this relationship are known, the third can be calculated.