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#### kdbmvp

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##### Calculate pH of solution
« on: March 06, 2019, 10:34:00 PM »

Hello,

calculate pH if [HS-] = 10-3 M and [S2-] = 10-6 M. So when HS- dissociates, we get:

HS- -> H+ + S2-. I calculated the [H+] concentration to be ~1.2*10-11. Which results in a pH of approximately 10, if you use that pH = -log[H+]. But does this make any sense? I mean, how can some (although very little) [H+] yield a basic solution? Given the very small amount of [H+] I'd rather think the solution is neutral..?
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#### Borek

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##### Re: Calculate pH of solution
« Reply #1 on: March 06, 2019, 11:44:53 PM »

You are told what are the equilibrium concentrations, and you have to find out at which pH ratio of concentrations [HS-]/[S2-] is 10-3/10-6. HS- is not a source of H+ here, pH is kept at whatever value it has by some other means (like some other buffer).

I mean, how can some (although very little) [H+] yield a basic solution? Given the very small amount of [H+] I'd rather think the solution is neutral..?

In general it just means you ignored water autodissociation. Not that it matters here, see above.
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#### kdbmvp

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##### Re: Calculate pH of solution
« Reply #2 on: March 07, 2019, 01:47:50 AM »

Thanks! But didnt I find the correct H+ concentration, and correspondingly the right pH?
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#### AWK

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##### Re: Calculate pH of solution
« Reply #3 on: March 07, 2019, 02:37:55 AM »

What value of pKa2 you used. Different sources give the values ​​from 12.2 to 15
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#### kdbmvp

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##### Re: Calculate pH of solution
« Reply #4 on: March 07, 2019, 04:21:18 AM »

I used a pKa,2 value of 13.9.
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#### AWK

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##### Re: Calculate pH of solution
« Reply #5 on: March 07, 2019, 06:00:31 AM »

And you got pH 10.9?
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#### kdbmvp

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##### Re: Calculate pH of solution
« Reply #6 on: March 07, 2019, 06:36:34 AM »

Yeah. I don't quite understand how the pH can be higher than 7?
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#### AWK

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##### Re: Calculate pH of solution
« Reply #7 on: March 07, 2019, 06:58:58 AM »

A concentration of HS- in H2S solution is only slightly greater than 10-7 with pH slightly below 7. Your concentration indicated very diluted hydrogen sulfide salt (eg KHS with c=10-3 M) and pH will be ~10 to 11.
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#### kdbmvp

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##### Re: Calculate pH of solution
« Reply #8 on: March 07, 2019, 08:50:55 AM »

So we have a basic solution, even though we have [H+] in solution? How is that possible?
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#### Borek

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##### Re: Calculate pH of solution
« Reply #9 on: March 07, 2019, 08:54:37 AM »

Yeah. I don't quite understand how the pH can be higher than 7?

I told you: pH is forced on the system by other things present in the solution.

The simplest approach here is to start with the dissociation constant and solve it for [H+]. You will have three knowns on the RHS and unknown on the LHS. Just plug and chug.
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#### kdbmvp

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##### Re: Calculate pH of solution
« Reply #10 on: March 07, 2019, 09:22:06 AM »

Alright, thanks for the *delete me* much appreciated.
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#### Corribus

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##### Re: Calculate pH of solution
« Reply #11 on: March 08, 2019, 05:26:52 AM »

Sulfide and bisulfide solutions are known to be strongly alkaline because they both react with water to produce hydroxide ions.

https://en.wikipedia.org/wiki/Sulfide
https://en.wikipedia.org/wiki/Bisulfide

Your treatment of HS- as an acid is probably why you are getting confused.
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