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For H 2 S ; K a 1 = 10 - 7 , K a 2 = 1 0 - 1 4 . A Saturated solution of H 2 S in 0.1 M H 2 S contains Mn 2 + , Co 2 + and Ag + at an original concentration of 0.01 M each. Determine the pH range for selective precipitation of these metal ions K s p ; M n S = 2.5 × 10 - 10 , C o S = 4 × 10 - 21 , A g 2 S = 6.3 × 10 - 50 Calculate difference in pH for precipitation of Mns and CoS.

Correct answer

5.4

Step-by-step solution

For H 2 S, H 2 S ⇌ 2 H + + S 2 - K a = K a 1 K a 2 = H + 2 S 2 - H 2 S ; H + 2 = 1 0 - 2 1 H 2 S S 2 - H + 2 = 1 0 - 2 2 S 2 - ⇒ [S 2 - ] for precipitation of Ag 2 S is K sp / Ag + 2 = 6 . 3 × 1 0 - 5 0 1 0 - 2 2 = 6 . 3 × 1 0 - 4 6 M i.e., too low value it will precipitate at any pH. ⇒ [S 2 - ] for precipitation of CoS ⇒ 4 × 1 0 - 2 1 1 0 - 2 = 4 × 1 0 - 1 9 ∴ H + = 1 0 - 2 2 4 × 1 0 - 1 9 = 1 . 5 8 × 1 0 - 2 M ; pH = 1.8 ⇒ [S 2 - ] for precipitation of MnS is 2 . 5 × 1 0 - 1 0 1 0 - 2 = 2 . 5 × 1 0 - 8 H + = 1 0

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