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# Ionic Product of water Watch

1. Hey all
I'm just having a problem with the ionic product of water formula.
I just can't seen to get my head around it!
So I know it is basically an equilibrium constant, for the equilibrium of water:
H2O <=> H+ and OH-
So it should be [H+][OH-]/[H2O]
(the twos are meant to be small)
But you are supposed to treat H2O as a constant, and it is hence removed from the equation. Except I don't understand why, it says "there is so little OH- and H+ that the conc of H2O is considered to have a constant value". But then surely this means you are considering OH- and H+ to be 0, which would mean that Kw is 0 too!!

I am soo confused!! If anyone can help me at all that would be amazing, I've just been going round in circles
Thanks
2. (Original post by meeee)
Hey all
I'm just having a problem with the ionic product of water formula.
I just can't seen to get my head around it!
So I know it is basically an equilibrium constant, for the equilibrium of water:
H2O <=> H+ and OH-
So it should be [H+][OH-]/[H2O]
(the twos are meant to be small)
But you are supposed to treat H2O as a constant, and it is hence removed from the equation. Except I don't understand why, it says "there is so little OH- and H+ that the conc of H2O is considered to have a constant value". But then surely this means you are considering OH- and H+ to be 0, which would mean that Kw is 0 too!!

I am soo confused!! If anyone can help me at all that would be amazing, I've just been going round in circles
Thanks
You are quite right that the equilibrium law gives:

kc = [H+][OH-]/[H2O]

But if water is massively in excess and effectively constant then we can define a DIFFERENT constant, kw, that considers only the hydrogen ions and hydroxide ions.

If you like, think of kw = kc * [H2O]
3. (Original post by charco)
You are quite right that the equilibrium law gives:

kc = [H+][OH-]/[H2O]

But if water is massively in excess and effectively constant then we can define a DIFFERENT constant, kw, that considers only the hydrogen ions and hydroxide ions.

If you like, think of kw = kc * [H2O]
omg thankyou soo much! that was just driving me insane!
Thaaaaanks!

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Updated: April 20, 2013
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