A solution contains 1.47×10 -2 M ammonium sulfide and 1.18×10 -2 M potassium hyd

Question

A solution contains 1.47×10-2 M ammonium sulfide and 1.18×10-2 M potassium hydroxide.
Solid iron(II) acetate is added slowly to this mixture.

What is the concentration of sulfide ion when hydroxide ion begins to precipitate?
[sulfide] = ____ M

Ksp values:

Bromides PbBr2 6.3 × 10-6
AgBr 3.3 × 10-13 Carbonates BaCO3 8.1 × 10-9
CaCO3 3.8 × 10-9
CoCO3 8.0 × 10-13
CuCO3 2.5 × 10-10
FeCO3 3.5 × 10-11
PbCO3 1.5 × 10-13
MgCO3 4.0 × 10-5
MnCO3 1.8 × 10-11
NiCO3 6.6 × 10-9
Ag2CO3 8.1 × 10-12
ZnCO3 1.5 × 10-11 Chlorides PbCl2 1.7 × 10-5
AgCl 1.8 × 10-10 Chromates BaCrO4 2.0 × 10-10
CaCrO4 7.1 × 10-4
PbCrO4 1.8 × 10-14
Ag2CrO4 9.0 × 10-12 Cyanides Ni(CN)2 3.0 × 10-23
AgCN 1.2 × 10-16
Zn(CN)2 8.0 × 10-12 Fluorides BaF2 1.7 × 10-6
CaF2 3.9 × 10-11
PbF2 3.7 × 10-8
MgF2 6.4 × 10-9 Hydroxides AgOH 2.0 × 10-8
Al(OH)3 1.9 × 10-33
Ca(OH)2 7.9 × 10-6
Cr(OH)3 6.7 × 10-31
Co(OH)2 2.5 × 10-16
Cu(OH)2 1.6 × 10-19
Fe(OH)2 7.9 × 10-15
Fe(OH)3 6.3 × 10-38
Pb(OH)2 2.8 × 10-16
Mg(OH)2 1.5 × 10-11
Mn(OH)2 4.6 × 10-14
Ni(OH)2 2.8 × 10-16
Zn(OH)2 4.5 × 10-17 Iodides PbI2 8.7 × 10-9
AgI 1.5 × 10-16 Oxalates BaC2O4 1.1 × 10-7
CaC2O4 2.3 × 10-9
MgC2O4 8.6 × 10-5 Phosphates AlPO4 1.3 × 10-20
Ba3(PO4)2 1.3 × 10-29
Ca3(PO4)2 1.0 × 10-25
CrPO4 2.4 × 10-23
Pb3(PO4)2 3.0 × 10-44
Ag3PO4 1.3 × 10-20
Zn3(PO4)2 9.1 × 10-33 Sulfates BaSO4 1.1 × 10-10
CaSO4 2.4 × 10-5
PbSO4 1.8 × 10-8
Ag2SO4 1.7 × 10-5 Sulfides CaS 8 × 10-6
CoS 5.9 × 10-21
CuS 7.9 × 10-37
FeS 4.9 × 10-18
Fe2S3 1.4 × 10-88
PbS 3.2 × 10-28
MnS 5.1 × 10-15
NiS 3.0 × 10-21
Ag2S 1.0 × 10-49
ZnS 2.0 × 10-25 Sulfites BaSO3 8.0 × 10-7
CaSO3 1.3 × 10-8
Ag2SO3 1.5 × 10-14

Explanation / Answer

The Ksp of Fe(OH)2 is 7.9*10-15. Determine [Fe2+] when sulfide begins to precipitate.

Ksp [Fe(OH)2] = [Fe2+][OH-]2

====> 7.9*10-15 = [Fe2+]*(1.18*10-2)2

====> [Fe2+] = (7.9*10-15)/(1.18*10-2)2 = 5.67*10-11 M

Determine [S2-] when [Fe2+] = 5.67*10-11 M.

Ksp [FeS] = [Fe2+][S2-]

====> 4.9*10-18 = (5.67*10-11)*[S2-]

====> [S2-] = (4.9*10-18)/(5.67*10-11) = 8.64*10-8 M (ans).

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