Question

A solution of $NH _{4} Cl$ and $NH _{3}$ has $pH =8.0$. Which of the following hydroxides may be precipitated when this solution is mixed with equal volume of $0.2\, M$ of metal ion.

• A. $B a(O H)_{2}\left(K_{s p}=1.1 \times 10^{-4}\right)$
• B. $M g(O H)_{2}\left(K_{s p}=3.5 \times 10^{-4}\right)$
• C. $F e(O H)_{2}\left(K_{s p}=8.1 \times 10^{-16}\right)$
• D. $Ca ( OH )_{2}\left(K_{s p}=2.1 \times 10^{-5}\right)$

Answer 1

Answer: (C)

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