Question

You are given an aqueous solution and are asked to analyze it for the presence (or absence) of Ag+(aq), Ca2+(aq), and/or Hg2+(aq) ions. There are no other metal ions in the solution.  You add aqueous hydrochloric acid (HCl) to the solution, and nothing appears to happen.  You add aqueous sulfuric acid (H2SO4) to the solution, and a white precipitate forms.  You remove (via filtration) the white precipitate, and then add aqueous hydrogen sulfide (H2S) to the solution that remains. A black precipitate forms. Based on these observations, which ions are present in the original solution? (a) Ag+(aq), Ca2+(aq), and Hg2+(aq) (b) only Ag+(aq) and Hg2+(aq) (c) only Ca2+(aq) and Hg2+(aq) (d) only Ag+(aq) and Ca2+(aq) (e) only Hg2+(aq)

Answer 1

Answer:

(c) only Ca2+(aq) and Hg2+(aq)

Explanation:

• In the first step, hydrochloric acid (HCl) is added to the solution. In this case the equilibrium that could take place is:

Ag⁺(aq) + Cl⁻(aq) ↔ AgCl(s)

But no precipitate was formed, so Ag⁺(aq) is absent.

• By adding H₂SO₄(aq) the next equilibrium that could take place is:

Ca⁺²(aq) + SO₄⁻²(aq)  ↔ CaSO₄(s)

A white precipitate was formed, so Ca⁺² is present in the solution.

• The following could take place after adding H₂S(aq):

Hg²⁺(aq) + S⁻² ↔ HgS(s)

A black precipitate formed, so Hg⁺² is present as well.