An example of a salt that can dissolve in water to form a solution with a pH of 7.

Analyze the question: We need a salt whose aqueous solution is neutral (pH = 7). This depends on the acid and base that formed the salt.

Concept: Salt hydrolysis

• Strong acid + strong base gives a neutral salt (pH ≈ 7).
• Strong acid + weak base gives an acidic salt (pH < 7).
• Weak acid + strong base gives a basic salt (pH > 7).

Solution:

| Option | Salt | Formed from | Solution | |---|---|---|---| | A | Sodium chloride, NaCl | HCl (strong acid) + NaOH (strong base) | Neutral, pH ≈ 7 | | B | Ammonium chloride, NH₄Cl | HCl (strong) + NH₃ (weak base) | Acidic, pH < 7 | | C | Ammonium tetraoxosulphate(VI), (NH₄)₂SO₄ | H₂SO₄ (strong) + NH₃ (weak) | Acidic, pH < 7 | | D | Sodium trioxocarbonate(IV), Na₂CO₃ | H₂CO₃ (weak acid) + NaOH (strong base) | Basic, pH > 7 |

Why they behave this way:

• In NaCl, neither Na⁺ nor Cl⁻ reacts with water, so H⁺ and OH⁻ stay balanced and the solution is neutral.
• In NH₄Cl and (NH₄)₂SO₄, NH₄⁺ hydrolyses: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺, which makes the solution acidic.
• In Na₂CO₃, CO₃²⁻ hydrolyses: CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻, which makes the solution basic.

Answer: Option A, sodium chloride.

Exam tip: Think "the strong one wins." If the acid and base are both strong, the salt is neutral. Otherwise the solution takes the character of the stronger parent: ammonium salts of strong acids are acidic, and sodium salts of weak acids (carbonates, ethanoates) are basic.