March 02, 2021, 02:15:46 PM
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Topic: why does the value of Kw(concentration of autoionized watermolecules) alwayshold  (Read 1361 times)

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Offline iScience

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Why does the equation     kw=[H3O+][OH-]=10-14M   always hold true?

the value 10-7M comes from the simple fact that this is the concentration of one of the two ions @ 25° C. and i understand that the concentration of one must equal the other for a regular sample of water [H3O+]=[OH-]. and i thought this was because in water, when an H2O ionizes, the same number of H2O's that got donated an H+ is the same number of H2O's that got their H+ stripped away and there are no other additional H+ ion donors, therefore they must be equal. However, for a solution of NaOH, why does the equation still hold? now there IS an additional source of OH ions. so why does  kw=[H3O+][OH-]=10-14M still hold?

Offline magician4

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because there is no such law that demands that H+ always has to be produced from former water  (and hence with an equal amount of OH- around)

the only law that applies here is the law of mass action , and this law only demands that there is a certain product  [H+] * [ OH-] always to be kept

...and this law never can be violated, and hence will hold, even in an aqueous solution of NaOH


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