What [H 3 O + ] must be maintained in a saturated H 2 S solution to precipitate Pb 2+ , but not Zn 2+ from a solution in which each ion is present at a concentration of 0.01 M ? (K sp of H 2 S = 1.1 × 10 –22 ; K sp of ZnS = 1.0 × 10 –21 ; 
= 3.3)
Text Solution
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For ZnS not to be precipitated from a solution of Zn 2+ and Pb 2+ , I P ZnS < K sp(ZnS)
[Zn 2+ ][S 2– ] < K sp of ZnS
[10 –2 ][S 2– ] < 1.0 × 10 –21 ∴ [S 2– ] < 10 –19 M
So, at [S 2– ] = 10 –19 M or less, no precipitation of ZnS will occur.
H 2 S 
2H + + S 2–
∴ [H + ] 2 [S 2– ] = 
= 1.1 × 10 –22 ∴ 
[10 –19 ] = 1.1 × 10 –22
= 11 × 10 –4 ∴ [H + ] min = 3.3 × 10 –2 M
Thus, if [H + ] = 3.3 × 10 –2 M or more, the precipitation of ZnS will not take place and only PbS will precipitate.
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