IB Chemistry SL

Revision Notes

8.1.8 The Ionic Product of Water

The Ionic Product of Water

pH of water

  • An equilibrium exists in water where few water molecules dissociate into proton and hydroxide ions

H2O(l) ⇌ H+(aq) + OH(aq)

  • The equilibrium constant for this reaction is:

The pH Scale equation

Kc x [H2O] = [H+] [OH]

  • Since the concentration the H+ and OH ions is very small, the concentration of water is considered to be a constant, such that the expression can be rewritten as:

Kw = [H+] [OH]

Where Kw (ionic product of water)      =         Kc x [H2O]

 =         10-14 mol2 dm-6 at 298K

  • The product of the two ion concentrations is always 10-14 mol2 dm-6
  • This makes it straightforward to see the relationship between the two concentrations and the nature of the solution:

[H+] & [OH] Table







Worked Example

What is the pH of a solution of potassium hydroxide, KOH(aq) of concentration 1.0 × 10−3 mol dm−3 ?

Kw = 1.0 × 10−14 moldm-6

   A. 3

   B. 4

   C. 10

   D. 11


The correct option is D.

    • Since Kw = [H+] [OH] , rearranging gives [H+]  = Kw ÷ [OH]
    • The concentration of  [H+] is (1.0 × 10−14) ÷ ( 1.0 × 10−3) = 1.0 × 10−11 mol dm−3
    • So the pH = 11


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