Chemistry Forums for Students > High School Chemistry Forum
Ksp Problem
(1/1)
Karakth:
I would be much obliged if someone could work this out and let me know if I am right or wrong. Thanks:
--- Quote ---In a saturated aqueous solution of H2S, [H+ ]2[S2- ] is 1.1x10-23 mol3dm-9. The solubility products of:
CdS = 3.6x10-29
FeS = 3.7x10-19
MnS = 1.4x10-15
NiS = 1.4x10-24
A solution contains each of the metal ions at a concentration of 0.1M and 0.25M HCl. The solution is saturated with H2S. Calculate which of the metals will be precipitated.
--- End quote ---
Right, my working:
First, I found the solubility of H2S in 0.25M HCl.
HCl --> H+ + Cl-
Ksp = [H+ ]2[S2- ]
H2S <--> 2H+ + S2-
Let solubility be s
[H+ ] = 2s + 0.25 = 0.25 (because s << 0.25, assume it negligible)
[S2- ] = s
Ksp = 1.1x10-23 mol3dm-9 (given)
1.1x10-23 = (0.25)2s
s = 1.1x10-23/(0.25)2 = 1.76x10-22
[S2- ] = 1.76x10-22moldm-3
Now I find the ionic products of the resulting solution and compare them to the given Ksps to see if a precipitate occurs.
Ionic Product (IP) of MS = 0.1 x 1.76x10-22 = 1.76x10-22
CdS: 1.76x10-22 > 3.6x10-29 --- Precipitate occurs
FeS: 1.76x10-22 < 3.7x10-19 --- No precipitate
MnS: 1.76x10-22 < 1.4x10-15 --- No precipitate
NiS: 1.76x10-22> 1.4x10-24 --- Precipitate occurs
And that's it...Phew. Thanks in advance to anyone who checks it. Also, this is a fine example of Ksp, if it's right :D
Borek:
1. [S2- ] - add space before last ]. That's some kind of glitch in the forum software.
2. What you are calculating is not a solubility of H2S, but equilibrium concentration due to acid dissociation.
3. There is something wrong with the numbers in your final stage of calculations - why do you compare Ksp of NiS and MnS? ;)
Karakth:
Heh yeah the comparision at the end is a mix-up of copy and paste numbers...I'll sort it out.
--- Quote ---2. What you are calculating is not a solubility of H2S, but equilibrium concentration due to acid dissociation.
--- End quote ---
Is it right to call this the solubility of H2S in 0.25M HCl?
Borek:
--- Quote from: Karakth on October 05, 2005, 09:39:17 AM ---Is it right to call this the solubility of H2S in 0.25M HCl?
--- End quote ---
No. Solubility of the H2S is much larger then the concentration of S2-. Solubility will be the sum of concentrations of H2S, HS- and S2-.
Navigation
[0] Message Index
Go to full version