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# K water Auto-ionisation of water watch

1. Hello guys,

I have a question; Why in all aqueuos solutions, the concentration of hydrogen ions * concentration of hydroxide ions is equal to 1*10^-14?

What if both concentrations are 0.1? 0.1 *0.1 is equal to 0.01.

2. If you start with pure water, i.e. no water molecules have dissociated, then 2H2O -> H3O+ + OH- until you reach a dynamic equilibrium in which 2H2O <-> H3O+ + OH- and [H+] and [OH-] are constant. They also have to be equal, as when H3O+ is created a OH- must also be created.

The position of this equilibrium is, as usual, sensitive to temperature. For forward reaction involves the breaking of a bond and is therefore endothermic. Increasing the temperature leads to an increase in both [H+] and [OH-].

At 20 oC it turns out that [H+] = [OH-] = 1 x 10^-7. Since Kw = [H+] [OH-] = (1 x 10^-7)^2 = 1 x 10^-14.

In a crazy world where water could be heated sufficiently to cause so many H-OH bonds to break, you could in theory have [H+] = [OH-] = 0.1 mol dm-3, in which case Kw would equal 0.01 mol2 dm-6.

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Updated: November 14, 2015
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