For which of the following can the metal ions be separated by adding a solution of hydrochloric acid (HCl(aq))?

Learn the solubility rules....

"All chlorides are soluble except for AgCl, PbCl2 and Hg2Cl2." Adding a source of chloride ions, Cl-, should precipitate one metal ion, but not the other.

(a) sodium nitrate and mercury(I) nitrate ..... precipitates Hg2Cl2 only
(b) calcium nitrate and strontium nitrate ...... no precipitate
(c) silver nitrate and mercury(I) nitrate ......... precipitates AgCl and Hg2Cl2 ... won't separate them
(d) iron(II) nitrate and aluminum nitrate ........ no precipitate

(a) True
Mercury(I) ions react with chloride ions in hydrochloric acid, to give mercury(I) chloride precipitate, while sodium ions do not react.
Hg²⁺(aq) + 2Cl⁻(aq) → Hg₂Cl₂(s)
After filtration, the filtrate contains sodium ions, and the residue is mercury(I) chloride solid which contains mercury(I) ions. Hence, sodium and mercury(I) ions can be separated.

(b) False
Both calcium and strontium ions do not react hydrochloric acid.
Hence, both calcium and strontium ions still mix in the solution after the addition of hydrochloric acid.

(c) False
Both silver ions in silver nitrate solution and mercury(I) ions in mercury(I) nitrate solution react with chloride ions in hydrochloric acid to give silver chloride and mercury(I) precipitates respectively.
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
Hg²⁺(aq) + 2Cl⁻(aq) → Hg₂Cl₂(s)
After addition of hydrochloric acid, the two cations exist in the mixture of two precipitates and are separated.


(d) False
Both iron(II) ions in iron(II) nitrate solution and aluminum ions in aluminum nitrate solution do not react with hydrochloric acid.
After addition of hydrochloric acid, the two cations still mix in the solution and are not separated.

The answer: (a) sodium nitrate and mercury(I) nitrate