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Offline 1stplace

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pH question
« on: April 12, 2008, 01:13:59 PM »
What is the pH of a solution with 0.122 M of CH3COONa, 0.500 M of CH3COOH acid, and hydrogen sulfide bubbled up to saturation (0.1 M)?

Acetic acid: Ka=1.75E-5
Hydrogen sulfide: Ka1=9.6E-8 Ka2=1.3E-14

I think this is supposed to be a simple calculation because the question leaves only a small amount of blank to do this question in. Can I assume that since there is a buffer in the solution (acetic acid/acetic salt), the acidic dissociation of H2S has no effect?

Thanks a lot

Offline Borek

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Re: pH question
« Reply #1 on: April 12, 2008, 02:54:09 PM »
Can I assume that since there is a buffer in the solution (acetic acid/acetic salt), the acidic dissociation of H2S has no effect?

What will be pH of the acetic buffer?

Once you are ready with this part, take a look at H2S dissociation - you should be able to calculate ratio of [H2S]/[HS-] from known pKa1 and pH. Does it look as if H2S dissociation is negligible, or not?
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Offline 1stplace

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Re: pH question
« Reply #2 on: April 12, 2008, 03:41:29 PM »
I got for the ratio of HS-/H2S as 0.0012. I know that that is pretty small but does this count as negligible?

Also, even if H2S dissociates into HS-, won't the buffer absorb some of that impact? This is because the buffer concentrations are much larger than HS- concentrations.

Offline Borek

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Re: pH question
« Reply #3 on: April 12, 2008, 05:43:37 PM »
I got for the ratio of HS-/H2S as 0.0012.

OK

Quote
I know that that is pretty small but does this count as negligible?

Compare amount of H+ from the H2S dissociation with that of H+ in the solution.

Quote
Also, even if H2S dissociates into HS-, won't the buffer absorb some of that impact? This is because the buffer concentrations are much larger than HS- concentrations.

It will for sure, question is - to what extent? Check if the pH will change once all H+ from H2S dissociation is used to protonate acetate.
« Last Edit: April 12, 2008, 06:07:37 PM by Borek »
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