Question

An aqueous solution of the residue obtained after treatment with aqua regia is treated with strong solution of potassium nitrate after neutralisation with $N{H}_{4}OH$ and then the solution is acidified with $C{H}_{3}COOH$.
A yellow precipitate so formed confirms the presence of:

• A. Zinc ions
• B. Cobalt ions
• C. Calcium ions
• D. Barium ions

Answer 1

The correct option is B Cobalt ions

Analysis of Group IV Cations
Cobalt sulphide dissolves in aqua regia in the same manner as nickel sulphide
When the aqueous solution of the residue obtained after treatment with aqua regia is treated with strong solution of potassium nitrate after neutralisation with $N{H}_{4}OH$ and the solution is acidified with $C{H}_{3}COOH$ , A yellow precipitate of the complex of cobalt named potassium hexanitritocobaltate (III) is formed

$CoS+HN{O}_3+3HCl\to CoC{l}_2+NOCl+S+2H_{2}O$

$CoC{l}_2+7KN{O}_2+2C{H}_{3}COOH\to \underset{\text{Potassium hexanitritocobalt(III)}\text{(Yellow precipitate)}}{K_3\left[Co\right(N{O}_2{)}_6]}+KCl+2C{H}_{3}COOK+NO+H_{2}O$