A metal that forms soluble trioxosulphate (IV) ion is

Analysis

The question asks us to identify which of the given metals forms a soluble trioxosulphate(IV) salt. The trioxosulphate(IV) ion is represented as $\text{SO}_3^{2-}$. Therefore, we are looking for a metal whose sulphite salt is soluble in water.

Concept: Solubility Rules

In chemistry, the solubility of salts follows specific general rules:

1. Group 1 Metals: All common salts of Group 1 elements (the alkali metals, such as Sodium $\text{Na}^+$, Potassium $\text{K}^+$, etc.) are soluble in water.
2. Sulphites ($\text{SO}_3^{2-}$): Most metal sulphites are insoluble in water, with the notable exception of those formed by Group 1 metals and ammonium ($\text{NH}_4^+$).
3. Barium ($\text{Ba}^{2+}$): Barium sulphite ($\text{BaSO}_3$) is known to be insoluble in water.
4. Aluminium ($\text{Al}^{3+}$) and Manganese ($\text{Mn}^{2+}$): These generally form insoluble sulphites or are unstable in aqueous solutions.

Step-by-Step Solution

1. Evaluate Option A (Barium): Barium forms $\text{BaSO}_3$. According to solubility tables, barium sulphite is insoluble in water.
2. Evaluate Option B (Potassium): Potassium is a Group 1 alkali metal. All salts of potassium, including potassium trioxosulphate(IV) ($\text{K}_2\text{SO}_3$), are highly soluble in water.
3. Evaluate Option C (Manganese): Manganese(II) sulphite is generally insoluble in water.
4. Evaluate Option D (Aluminium): Aluminium sulphite is not a standard stable soluble salt in aqueous solution.

Since Potassium belongs to Group 1, it satisfies the rule that all its common salts are soluble.

Conclusion

This aligns with the chemical principle that all Group 1 metal salts are soluble.

Exam Tip

Remember the acronym "SPA" for solubility: Sodium, Potassium, and Ammonium salts are always soluble in water, regardless of the anion (nitrate, chloride, sulphate, or sulphite) attached to them. If you see these elements in a solubility question, they are almost always the correct answer!