[Karakth]

I would be much obliged if someone could work this out and let me know if I am right or wrong. Thanks:

In a saturated aqueous solution of H₂S, [H⁺ ]²[S^2- ] 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.

Right, my working:

First, I found the solubility of H₂S in 0.25M HCl.

HCl --> H⁺ + Cl^-

K_sp = [H⁺ ]²[S^2- ]
H₂S <--> 2H⁺ + S^2-

Let solubility be s

[H⁺ ] = 2s + 0.25 = 0.25 (because s << 0.25, assume it negligible)
[S^2- ] = s
K_sp = 1.1x10^-23 mol³dm^-9 (given)

1.1x10^-23 = (0.25)²s
s = 1.1x10^-23/(0.25)² = 1.76x10^-22

[S^2- ] = 1.76x10^-22moldm^-3

Now I find the ionic products of the resulting solution and compare them to the given K_sps 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 K_sp, if it's right

[Borek]

1. [S^2- ] - 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 K_sp 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.

2. What you are calculating is not a solubility of H2S, but equilibrium concentration due to acid dissociation.

Is it right to call this the solubility of H₂S in 0.25M HCl?

[Borek]

Is it right to call this the solubility of H₂S in 0.25M HCl?

No. Solubility of the H₂S is much larger then the concentration of S^2-. Solubility will be the sum of concentrations of H₂S, HS^- and S^2-.