Chemistry 2008 JAMB Past Questions - Question 35: When a solution of ammonium trioxocarbonate(IV) is added to a solution of an unknown salt, a white precipitate which is soluble in dilute hydrochloric acid but insoluble in ethanoic acid is formed. This indicates the presence of

Correct Answer

A

Explanation

The information provided suggests the presence of the **Ca²+ ion**. Here's the reasoning:

1. **Solubility clues:**
    * **Soluble in dilute hydrochloric acid:** This eliminates Na⁺ and K⁺ as their chlorides are also soluble in dilute acids.
    * **Insoluble in ethanoic acid:** This eliminates Zn²⁺ as zinc acetate is soluble in ethanoic acid.

2. **Ammonium trioxocarbonate(IV) reaction:**
    * Ammonium trioxocarbonate(IV) (ammonium carbonate), when dissolved in water, dissociates into ammonium (NH₄⁺) and carbonate (CO₃²⁻) ions.
    * The carbonate ion can react with certain metal cations to form insoluble carbonate salts.

3. **Matching the clues:**
    * Among the remaining options, only calcium carbonate (CaCO₃) is insoluble in ethanoic acid.
    * Additionally, calcium carbonate reacts with dilute hydrochloric acid, dissolving to form calcium chloride (CaCl₂) and releasing carbon dioxide (CO₂) gas:

      CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l)

Therefore, based on the solubility behavior and the reaction with ammonium carbonate, the unknown salt **most likely contains the Ca²+ ion**.

It's important to note that confirming the presence of a specific ion definitively might require additional tests or information about the expected salt in the unknown solution. However, based on the clues provided, Ca²+ is the most likely candidate.