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Question

The solubility products of Al(OH)3 and Zn(OH)2 are 8.5×1023 and 1.8×1014 at room temperature. If the solution contains Al3+ and Zn2+ ions, the ion first precipitated by adding NH4OH is:

A
Al3+
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B
Zn2+
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C
Both
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D
None of these
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Solution

The correct option is B Zn2+
Al(OH)3Al3++3OH

Ks=[Al3+][OH]3

=S1×(3S1)3

=27S41 but Ks=8.5×1023

S1=1.32×106.

Zn(OH)2Al2++2OH

Ks=[Zn2+][OH]2

=S2×(2S2)2

=4S32 but Ks=1.8×1014

S2=1.64×105.

So Zn2+ will precipitate first.

Hence, the correct option is B

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