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Topic: autoionization of water  (Read 2613 times)

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Offline Billqaz3

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autoionization of water
« on: August 24, 2015, 03:21:25 PM »
Im trying to figure out the autoionization of water. Every textbooks says that in acid base calculations you have to take into account the autoionization of water but all the examples say that the H+ contributed by water is negligible. I was wondering in what situation would the autoionization of water be considered.

This is just my guess as to how this works but lets say we had a solution of HCl at 1x10^-7 M since the concentrating of H+ in water at 25 C is now significant this would cause a shift to the right in the equilibrium equation of water.

H2O <---> H30        + OH-                       Kw=[H3O][OH]
----           2x10^-7     1x10^-7         1x10^14=[2x10^-7-x][1x10^-7-x]
----           -x              -x   
----           

Offline Borek

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Re: autoionization of water
« Reply #1 on: August 24, 2015, 05:27:32 PM »
Try to calculate pH of 10-8 M HCl solution.
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Offline Billqaz3

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Re: autoionization of water
« Reply #2 on: August 24, 2015, 06:48:30 PM »
The pH is 8 I don't understand how adding an acid to water can result in a basic solution

Offline Borek

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Re: autoionization of water
« Reply #3 on: August 25, 2015, 02:42:11 AM »
The pH is 8 I don't understand how adding an acid to water can result in a basic solution

That's exactly why you can't ignore the water autoionization. When the amount of acid is low, H+ contributed by water can no longer be ignored.
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Offline Enthalpy

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Re: autoionization of water
« Reply #4 on: August 27, 2015, 11:44:38 AM »
Water contains an amount of [H3O]*[OH] (which depends on the temperature).
When really pure, neutrality means [H3O]=[OH]=1e-7. A smaller amount of acid or base just adds ions to this minimum amount - compute it from the known product and from the electric neutrality.

Because water's autoionization is really small, it rarely matters. Tiny amounts of impurities overshadow it. Semiconductor fabs use ultrapure water that is deionized by ion-exchange resins (or was, looong ago), and the theoretical water resistivity is attained exceptionally, under the best circumstances.

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