Suppose hydrogen sulfide is added to a solution that is 0.10 m in cu2+, pb2+, and ni2+ such that the concentration of h2s is 0.10 m. when the ph of the solution is adjusted to 1.00, a precipitate forms. what is the composition of the precipitate? h2s(aq) + 2h2o(l) 2h3o+(aq) + s2–(aq); kc = 1.1 × 10–20 salt ksp cus 6.0 × 10–36 pbs 2.5 × 10–27 nis 3.0 × 10–19
a. nis only
b. cus, pbs, and nis
c. cus and pbs
d. cus only
e. pbs and nis

C. CuS and PbS  

Explanation:

1. Calculate [H₃O⁺]

pH = 1.00  

2. Calculate the concentration of S²⁻

H₂S + 2H₂O ⇌ S²⁻ + 2H₃O⁺; Kc =  1.1 × 10⁻²⁰

3. Calculate Qsp for the sulfides

(a) CuS

CuS ⇌ Cu²⁺ + S²⁻; Ksp = 6.0 × 10⁻³⁶

Qsp = [Cu²⁺][S²⁻] = (0.10)(1.0 × 10⁻¹⁹) = 1.0 × 10⁻²⁰

Qsp > Ksp, so a precipitate of CuS will form.

(b) PbS

PbS ⇌ Pb²⁺ + S²⁻; Ksp = 2.5 × 10⁻²⁷

Qsp = [Pb²⁺][S²⁻] = (0.10)(1.0 × 10⁻¹⁹) = 1.0 × 10⁻²⁰

Qsp > Ksp, so a precipitate of PbS will form.

(iii) NiS

NiS ⇌ Ni²⁺ + S²⁻; Ksp = 3.0 × 10⁻¹⁹

Qsp = [Ni²⁺][S²⁻] = (0.10)(1.0 × 10⁻¹⁹) = 1.0 × 10⁻²⁰

Qsp < Ksp, so a precipitate will not form.

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