#1a... A solution is saturated with H 2 S at a concentration of 0.10 M, and is 4
ID: 952014 • Letter: #
Question
#1a... A solution is saturated with H2S at a concentration of 0.10 M, and is 4.0 × 104 M in each of the metal cations: Mn2+, Fe2+, Ni2+, Co2+, and Zn2+. Under these conditions, what pH is needed to precipitate as much of the Zn2+as possible without precipitating the Fe2+, Co2+, Ni2+, and Mn2+?
General Equilibrium:
MS(s) + 2H3O+(aq) M2+(aq) + 2 H2O(l) + H2S(aq); Kspa
Metal Sulfide
Kspa
MnS
3 × 1010
FeS
6 × 102
CoS
3
NiS
8 × 101
ZnS
3 × 102
pH = 1.4
pH = 2.2
pH = 2.4
pH = 3.6
pH = 7.4
#1B ... A solution is 4.0 × 103 M in Co2+ and 0.10 M in H2S. At what pH will CoS (Kspa = 3) first begin to precipitate from solution?
General Equilibrium:
MS(s) + 2H3O+(aq) M2+(aq) + 2 H2O(l) + H2S(aq); Kspa
pH > 3.9
pH > 1.5
pH > 6.8
pH > 1.9
pH > 4.4.
Metal Sulfide
Kspa
MnS
3 × 1010
FeS
6 × 102
CoS
3
NiS
8 × 101
ZnS
3 × 102
Explanation / Answer
precipitation occurs only whe the ionoc prodict exceedes the solubility product
for precipitatio of metal
the Ksp =( [M+] X [S2-])/ [H+]2
thus subs tituting value we have
3 X 10-2 = (4X10-4 X 0.1) / ([H+])2
[H+] =0.0365
pH = 1.4 , thus at pH this pH only ZnS precpitates.
b)
Ksp =( [M+] X [S2-])/ [H+]2
similarly
substituting values for CoS
3 = (4X10-3 X 0.1) / ([H+])2
[H+] = 0.115
pH = 1.9
thus at this pH and and more only CoS starts precipitating.
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