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Topic: Acid Base equilibrium, buffer, saline idrolisis  (Read 3324 times)

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Offline ultrashogun

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Acid Base equilibrium, buffer, saline idrolisis
« on: January 24, 2007, 03:00:43 PM »
Hi, Id like to ask some opinions regarding a problem I have. Im normally pretty good with these problems, but this one bugs me.

Calculate the pH of a solution obtained by mixing 100mL of H2S 0.1M with a) 200mL KOH 0.05M; b) 300ml KOH 0.05M

The constants of dissociation are given: Ka1 = 1.1 x 10^-7 and Ka2 = 10^-14

For a) I first note that the two compounds are equimolar, so the reaction H2S + KOH --> HS- + H20 + K+ proceeds untill the total consumption of both compounds, after which I have a solution of the conjugate base, for which I can calculate the pH by means of [OH] = (Kb x Cb)^1/2, after finding Kb with Kb = Kw / Ka1 and adjusting the concentration to the new volume.

My real problem lies in part b).

I now have a excess of KOH in respect to H2S. Normally I would argue that the reaction HS- + KOH --> H20 + S2- + K+ would then proceed towarss the consumption of KOH. If this is the case then I wouldnt know how to get the pH because in addition to the equilibrium HS- / S2-, I also have the an equilibrium with H2S / HS-.

Offline Borek

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Re: Acid Base equilibrium, buffer, saline idrolisis
« Reply #1 on: January 24, 2007, 05:57:41 PM »
I now have a excess of KOH in respect to H2S. Normally I would argue that the reaction HS- + KOH --> H20 + S2- + K+ would then proceed towarss the consumption of KOH. If this is the case then I wouldnt know how to get the pH because in addition to the equilibrium HS- / S2-, I also have the an equilibrium with H2S / HS-.

Yep. But you may at first assume that H2S/HS- equilibrium is neglectable and calculate pH, and than using calculated pH value check if the assumption holds.

How to check? Think in terms of Henderson-Hasselbalch equation - what does it tell you about ratio of acid and conjugate base when pH is several units from pKa?

Note that you sometimes can't use HH to calculate pH (when you don't know what are equilibrium concentrations of acid/base), but it still holds when you put known pKa and pH to find out acid/base ratio.
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