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Topic: pH at the second stoichiometric point  (Read 3237 times)

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Offline LHM

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pH at the second stoichiometric point
« on: April 07, 2011, 10:48:30 PM »
For the second stoichiometric point of H2S2O3, basically I did some calculations and got that for S2O32- + H2O ::equil:: HS2O32- + OH-, the [OH-]=4.30*10-8, which is what the answer key had. I didn't have any problems with the calculations up to that point, it was after this where I have questions.

Basically, the answer key says that since 4.30*10-8 < [OH-]water, so the pH=7.0 at the second stoichiometric point. This seems a bit odd to me because if this were the case, is the pH of the second stoichiometric point of all diprotic acids be pH=7.0 or higher? And also, If you calculated [H+] from [OH-]=4.30*10-8, shouldn't it give a [H+]>10-7, so pH<7.0?

Offline AWK

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Re: pH at the second stoichiometric point
« Reply #1 on: April 08, 2011, 02:00:48 AM »
pH=7 is a crude approximation only. For exact calculations you should take into account also Kw. The pH will be minutely higher.
AWK

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