Question

The only cations present in a slightly acidic solution are $ Fe^{3+},Zn^{2+} $ and $ Cu^{2+} $ The reagent that when added in excess to this solution would identify and separate $ Fe^{3+} $ in one step is

• A. $ 2M\,HCl $
• B. $ 6M\,NH_{3} $
• C. $ 6M\,NaOH $
• D. $ H_{2}S\,gas $

Answer 1

Answer: (B)

Cupric ion reacts with excess of ammonia solution to give a deep blue color of tetra ammine cupric ion.
$Cu ^{2+}+4 NH _{4} OH \rightarrow \underset{\text { Deep blue color }}{\left[ Cu \left( NH _{3}\right) 4\right]^{2+}}+4 H _{2} O$
$Zn$ (II) ions react with ammonia to form white gelatinous $ppt$ of $Zn ( OH )_{2}$ which is soluble in excess ammonia.
$Zn ^{2+}+2 NH _{3}+2 H _{2} O \rightarrow Zn ( OH )_{2} \downarrow+2 NH _{4}^{+} $
$ Zn ( OH ) \downarrow 4 NH _{3} \rightarrow\left[ Zn \left( NH _{3}\right)_{4}\right]^{2+}+2 OH ^{-}$
When the above reactions are not observed, $Fe ^{3+}$ ion is confirmed.