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Question: Answered & Verified by Expert
A solution of $\mathrm{NH}_{4} \mathrm{Cl}$ and $\mathrm{NH}_{3}$ has $\mathrm{pH}=8.0 .$ Which of the following hydroxides may be precipitated when this solution is mixed with equal volume of 0.2 M of metal ion.
ChemistryIonic EquilibriumJEE Main
Options:
  • A $\mathrm{Ba}(\mathrm{OH})_{2}\left(\mathrm{~K}_{\mathrm{sp}}=1.1 \times 10^{-4}\right)$
  • B $\mathrm{Mg}(\mathrm{OH})_{2}\left(\mathrm{~K}_{\mathrm{sp}}=3.5 \times 10^{-4}\right)$
  • C $\mathrm{Fe}(\mathrm{OH})_{2}\left(\mathrm{~K}_{\mathrm{sp}}=8.1 \times 10^{-16}\right)$
  • D $\mathrm{Ca}(\mathrm{OH})_{2}\left(\mathrm{~K}_{\mathrm{sp}}=2.1 \times 10^{-5}\right)$ ).
Solution:
2609 Upvotes Verified Answer
The correct answer is: $\mathrm{Fe}(\mathrm{OH})_{2}\left(\mathrm{~K}_{\mathrm{sp}}=8.1 \times 10^{-16}\right)$
$\mathrm{pH}=8, \mathrm{pOH}=6 ;\left[\mathrm{OH}^{-}\right]=10^{-6} \mathrm{M}$

Ionic product of $\mathrm{Fe}(\mathrm{OH})_{2}=0.2 \times\left(1 \times 10^{-6}\right)^{2}$

$=2 \times 10^{-13}>\mathrm{K}_{\mathrm{sp}}\left(=8.1 \times 10^{-16}\right)$

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