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The Ionic Product of Water, Kw 



Acids & Bases

AQA Content

Use Kw to calculate the pH of a strong base from its concentration.

Specification Notes

Water is slightly dissociated.
Kw is derived from the equilibrium constant for this dissociation: Kw = [H+][OH–]
The value of Kw varies with temperature.


The equilibrium:

Is usually simplified to:

According to the equilibrium law:

The concentration of the water essentially remains constant (remember it only slightly dissociates) so that term may be removed. We can now form a new equilibrium constant, Kw - the ionic product of water.

At 25ºC (298K) the value of Kw is...
When dissociated...

Temperature effect on Kw

For this equilibrium...
Energy is required to break the bonds in water, so it is endothermic in the forward direction.

If we consider this in terms of Le Chatelier, if temperature increases then the forward reaction will be favoured and the equilibrium will move to the right, the concentration of ions increases and so Kw must increase.

Since the hydrogen ion concentration increases with increasing temperature, the value for pH will decrease and the water will be more acidic than at 25°C.

pH of strong bases

The Kw equation can be used to calculate the pH of bases...
Just include the hydroxide ion concentration in the kw equation.

What is the pH of a solution of NaOH with a concentration of 0.15 M at 25°C.?

Strong bases dissociate fully when dissolved:
The concentration of hydroxide ions is the same as that of NaOH


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