An aqueous solution of a metal bromide, MBr 2 (0.05 M) is saturated with H 2 S. Calculate the minimum pH at which the metal sulphide will be precipitated. Concentration of H 2 S in a saturated solution = 0.1M. [K a1 of (H 2 S) = 10 –7 , K a2 of (H 2 S) = 1.2 × 10 –13 , K sp for MS = 6 × 10 –21 ]
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Sol. For precipiatation of MS, K sp of (MS) = [M +2 ] [S –2 ] = 6 × 10 –21
⇒ [S 2– ] req = 
= 120 × 10 –21 = 1.2 × 10 –19 M
Precipitation of MS will start when H 2 S will provide atleast 1.2 × 10 –19 M S –2 .
For H 2 S, H 2 S 
2 H + + S –2 ;K = K a1 .K a2 = 
10 –7 × 1.2 × 10 –13 = 
; [H + ] 2 = 10 –2
So, [H + ] = 0.1 M ∴ pH = 1
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