Question

A solution, containing 0.01 M Zn+2 and 0.01 M
Cu2+ is saturated by passing H,S gas. The S-2
concentration is 8.1 x 10-21 M, Ksp for ZnS and
CuS are 3.0 x 10-22 and 8.0 x 10-36 respectively.
Which of the following will occur in the solution:-
(1) ZnS will precipitate
(2) CuS will precipitate
(3) Both ZnS and Cus will precipitate
(4) Both Zn2+ and Cu2+ will remain in the solution​

Answer 1

Answer:

(2) CuS will precipitate

Explanation:

Zn2+ concentration = 0.01M

Cu2+ concentration = 0.01M

S2- conecntration = 8.1 × 10 ⁻²¹M

Ksp (ZnS) = 3 × 10⁻²²

Ksp (CuS) = 8.0 × 10⁻³⁶

At first,

ZnS⇆Zn²⁺ + S²⁻

Kip (ZnS) = Zn²⁺ × S²⁻

               = 0.01 × 8.1 × 10⁻²¹

               = 8.1 × 10⁻¹⁹

so, Kip (ZnS) is almost same to Ksp (ZnS)

similarly,

CuS⇆Cu²⁺ + S²⁻

Kip (CuS) = Cu²⁺ × S²⁻

               = 0.01 × 8.1 × 10⁻²¹

               = 8.1 × 10⁻¹⁹

here,  Kip (CuS) is greater than Ksp (CuS)

so, According to precipitation law CuS will precipitate in a bulk amount but there will be a little precipitation of ZnS as well.