Q17 / 125 · BITSAT 2020 · Chemistry

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.

  • A
    $\mathrm{Ba}(\mathrm{OH})_{2}\left(\mathrm{~K}_{\mathrm{sp}}=1.1 \times 10^{-4}\right)$
  • B
    $\mathrm{Mg}(\mathrm{OH})_{2}\left(\mathrm{~K}_{\mathrm{sp}}=3.5 \times 10^{-4}\right)$
  • Correct answer
    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

$\mathrm{pH}=8, \mathrm{pOH}=6 ;\left[\mathrm{OH}^{-}\right]=10^{-6} \mathrm{M}$ Ionic product of $\mathrm{Fe}(\mathrm{OH})_{2}=0.2 \times\left(1 \times 10^{-6}\right)^{2}$ $=2 \times 10^{-13}>\mathrm{K}_{\mathrm{sp}}\left(=8.1 \times 10^{-16}\right)$

Hence, the correct answer is option: 'C'.

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