Solubility product of Al(OH)3 and Zn(OH)2 are 8.5×10−23 and 1.8×10−14 respectively. If both Al3+ and Zn2+ ions are present in a solution, which one will precipitate first on addition of NH4OH?
A
Al(OH)3
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B
Zn(OH)2
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C
Mg(OH)2
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D
Ca(OH)2
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Solution
The correct option is AAl(OH)3 The value of solubility product for Al(OH)3 is 8.5×10−23.
It is much lower than the value of the solubility product for Zn(OH)2 which is 1.8×10−14.
Hence, when both Al3+ and Zn2+ ions are present in the solution and as NH4OH is added, Al(OH)3 will precipitate first as the solubility product of Al(OH)3 is quite low.
Hence, its ionic product will exceed its solubility product at a very low concentration of OH− ions.