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Topic: Calculating concentration from K and PH  (Read 3870 times)

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Offline Zoloft

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Calculating concentration from K and PH
« on: May 26, 2009, 01:11:33 AM »
Sulfide forms many salts with widely varying solubilities.  We have used these varying solubilities to selectively precipitate metal ions in our qual scheme.  The key to this selective precipitation is the basicity of S2-, which allows its concentration to be controlled by controlling the pH of the solution.  Sulfide forms the acid H2S, for which the following equilibrium expression can be written:

[H+]2[S2-] / [H2S] = K = 1.32x10-20
To form sulfide precipitates, the solution is saturated with H2S, which produces a 0.10M concentration of H2S.  For a saturated solution, the equilibrium expression becomes:

[H+]2[S2-] = 1.32x10-21

1) Calculate the [S2-] in a saturated solution of H2S where the pH is 1.00. (2pts)

2) Calculate the [S2-] in a saturated solution of H2S where the pH is 9.00. (2pts)

3)  Consider a solution that contains 1.0 x 10-3M Mn2+ and 1.0 x 10-3M Cu2+, is saturated with H2S, and has a pH of 2.00.  Under these conditions, does either CuS (Ksp = 8.5x10-45) or MnS (Ksp=2.3x10-13) form? (4pts)

4) For which separations in the qual scheme do you rely on the differing solubilities sulfide complexes? (2pts)

How exactly would I calculate the pH of [S2-] from the K value and the PH?! I'm a bit confused, since I'm doing a General Unknown laboratory experiment and PH control is crucial to the separation of several ions.

Hints are appreciated, since I'm rather stumped...

Offline Train

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Re: Calculating concentration from K and PH
« Reply #1 on: May 26, 2009, 02:08:29 AM »
How exactly would I calculate the pH of [S2-] from the K value and the PH?!

Maybe it's just too late at night, but I don't understand this question.  How would you calculate pH from K and pH?  Doesn't that mean you already know the pH?

To answer the first two questions I would use [H+]2[S2-] = 1.32x10-21 and just plug in the appropriate [H+].

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