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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: iScience on July 21, 2013, 09:46:58 AM

Title: why does the value of Kw(concentration of autoionized watermolecules) alwayshold
Post by: iScience on July 21, 2013, 09:46:58 AM

Why does the equation     k_w=[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  k_w=[H₃O⁺][OH^-]=10^-14M still hold?

Title: Re: why does the value of Kw(concentration of autoionized watermolecules) alwayshold
Post by: magician4 on July 21, 2013, 02:57:20 PM

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


regards

ingo