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H 2 S is bubbled through 0 . 2 M   NaCN solution which is 0 . 02 M in each Ag CN 2 ( aq) - 1 and Cd CN 4 ( aq) 2 - . Determine which sulphide will precipitate first. K sp Ag 2 S = 1 0 - 5 0 ,  K sp CdS = 7 × 1 0 - 2 8 K f  Ag  CN 2 - 1 = 2 . 5 × 1 0 2 0 ,  K f  Cd CN 4 aq 2 - = 1 0 1 7 . S 2 - ion concentration for Ag 2 S and CdS may be reported as:

Options

  1. A2 . 5 × 10 - 9 M ,   5 . 6 × 10 - 12 M
  2. B5 . 0 × 10 - 9 M ,   11 . 2 × 10 - 12 M
  3. C2 . 5 × 10 - 6 M ,   5 . 6 × 10 - 9 M
  4. D2 . 5 × 10 - 5 M ,   5 . 6 × 10 - 7 M

Correct answer

A. 2 . 5 × 10 - 9 M ,   5 . 6 × 10 - 12 M

Step-by-step solution

2 Ag CN 2 aq - 1 ⇌ 2 Ag aq + + 4 CN aq - 1 ; 1 K f 2 AgCN 2 - 1 2 Ag + + S aq 2 - ⇌ Ag 2 S s ; 1 K sp Ag 2 S ---------------------------------------------------------------------------------------------------------- 2 Ag CN 2 aq - 1 + S aq 2 - ⇌ Ag 2 S s + 4 CN aq - 1 ; 1 K sp Ag 2 s K f 2 Ag CN 2 - 1 = K 1 K 1 = 1 0 + 5 0 6 . 2 5 × 1 0 4 0 = 1 . 6 × 1 0 9     K 1 = 1 K sp Ag 2 S ·   K f 2  Ag CN 2 - 1 1 . 6 × 1 0 9 = CN - 1 4 Ag CN 2 - 1 2 S 2 - = 0 .

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