A solution contains a mixture of 0.01 M Zn 2+ and 0.01 M Mn 2+ ions and is saturated with H 2 S (a saturated solution of H 2 S being 0.1 M). From the following statements, indicate the correct one – (Given :k sp of ZnS = 1.0 × 10 –22 , k sp of MnS = 5.6 × 10 –16 , [H + ] 2 [S –2 ] in saturated solution = 1.0 × 10 –22 )
Text Solution
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(a, b)
K sp (MnS) = 5.6 × 10 –16 = [Mn 2+ ] [S –2 ]
⇒ [S –2 ] = 5.6 × 10 –14 (For precipitation of MnS)
Now [H + ] 2 [S –2 ] = 1.0 × 10 –22 ⇒ [H
+ ] = 
= 4.2 × 10 –5 M
⇒ pH = 5 – log 4.2 = 5 – 0.6232 = 4.38
At this pH, [Zn 2+ ] = 
= 1.8 × 10 –9 M
Now K sp (CdS) < K sp (ZnS) < K sp (MnS) So ease of precipitation of ions is in order of Cd 2+ > Zn 2+ > Mn 2+ .
Again higher [H + ] i.e. lower [S –2 ] ion concentration Cd 2+ is most easily precipitated so it needs least [S –2 ] conc. hence maximum [H + ] so order of required [H + ] conc. is Mn 2+ < Zn 2+ < Cd 2+
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