Question

What is the maximum concentration of equimolar solutions of $FeSO _{4}$ and $Na _{2} S$ so that when mixed in equal volumes, there is no precipitation of $FeS$ ? $\left( K _{ sp }\right.$ of $FeS$ $\left.=6.25 \times 10^{-18}\right)$

• A. $5 \times 10^{-9} M$
• B. $5 \times 10^{-8} M$
• C. $5 \times 10^{-18} M$
• D. $5 \times 10^{9} M$

Answer 1

Answer: (A)

$s ^{2}=6.25 \times 10^{-18}$
$s =2.5 \times 10^{-9}$
$M =2 s=2 \times 2.5 \times 10^{-9}$
$=5.0 \times 10^{-9} M$