Question

A suspension containing insoluble substances $ZnS$, $CuS$, $HgS$, $A{{g}_{2}}S$ and $FeS$, is treated with 2N $HCl$. On filtering, the filtrate contained appreciable amounts of which one of the following groups?
(A) Zinc and mercury
(B) Silver and iron
(C) Copper and mercury
(D) Zinc, copper and iron

Answer 1

The question says a suspension has insoluble sulphate salts of zinc, copper, mercury, silver and iron. These salts are treated with 2N HCl. Think about the salts which will get completely dissolved in hydrochloric acid. The salts which get completely dissolved in acid will pass through the filter paper and will be present in the filtrate.

Complete step by step solution:
- Let’s compare the reactivity of metals given in the question.
- According to the electrochemical series, zinc, iron and copper have higher oxidizing potentials than silver and mercury. Therefore, zinc, iron and copper react with mineral acids and get dissolved.
- When a suspension containing insoluble substances $ZnS$, $CuS$, $HgS$, $A{{g}_{2}}S$ and FeS, is treated with 2N $HCl$, zinc, copper and iron, being soluble in hydrochloric acid, will dissolve in dilute acid.
- Mercury sulphide and silver sulphide are not soluble in 2N $HCl$ and so, will not get dissolved in $HCl$.
- Once this solution is filtered through a filter paper, the residue on the filter paper will have HgS and $A{{g}_{2}}S$. The filtrate will contain zinc ions, iron ions and copper ions which are in dissolved form.
- Therefore, the filtrate will contain appreciable amounts of zinc, iron and copper.

Therefore, the correct answer is option (D).

Note: Remember zinc, iron and copper are more prone to corrosion in the presence of mineral acids like hydrochloric acid, sulphuric acid, etc. Iron has very high oxidizing potential, so it also gets oxidized in the presence of moisture to form rust. Mercury and silver are very stable in the form of sulphide salts.