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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: LHM on April 07, 2011, 10:48:30 PM

Title: pH at the second stoichiometric point
Post by: LHM on April 07, 2011, 10:48:30 PM

For the second stoichiometric point of H₂S₂O₃, basically I did some calculations and got that for S₂O₃^2- + H₂O ::equil:: HS₂O₃^2- + 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?

Title: Re: pH at the second stoichiometric point
Post by: AWK on April 08, 2011, 02:00:48 AM

pH=7 is a crude approximation only. For exact calculations you should take into account also K_w. The pH will be minutely higher.