Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: kdbmvp on March 07, 2019, 04:34:00 AM
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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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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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Thanks! But didnt I find the correct H+ concentration, and correspondingly the right pH?
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What value of pKa2 you used. Different sources give the values from 12.2 to 15
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I used a pKa,2 value of 13.9.
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And you got pH 10.9?
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Yeah. I don't quite understand how the pH can be higher than 7?
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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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So we have a basic solution, even though we have [H+] in solution? How is that possible?
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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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Alright, thanks for the *delete me* much appreciated.
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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.