An example of an acid salt is

Analyze the question: We are asked to identify an acid salt. This is a salt formed when an acid is only partially neutralized, so it still contains replaceable hydrogen ion(s).

Concept:

• Normal salt: all replaceable H⁺ of the acid have been replaced by a metal or ammonium ion (e.g. NaCl, Na₂SO₄).
• Acid salt: only some of the replaceable H⁺ have been replaced, so the salt still has ionizable H (e.g. NaHCO₃, NaHSO₄).
• Basic salt: the base is only partially neutralized, so the salt still contains OH⁻ (e.g. Mg(OH)Cl).

Solution, option by option:

• A. CH₃COONa (sodium ethanoate): The acid, ethanoic acid, has only one replaceable H, and it has been replaced by Na. This is a normal salt.
• B. Mg(OH)Cl: It contains an OH⁻ group, which comes from incomplete neutralization of the base Mg(OH)₂. This is a basic salt.
• C. NaHSO₄ (sodium hydrogen sulphate): Sulphuric acid, H₂SO₄, has two replaceable H atoms. Only one has been replaced by Na, so one H remains. This is an acid salt. H₂SO₄ + NaOH → NaHSO₄ + H₂O
• D. (NH₄)₂SO₄: Both H atoms of H₂SO₄ have been replaced by NH₄⁺. This is a normal salt.

Answer: The acid salt is NaHSO₄, option C.

Exam tip: Look for a leftover H written in the formula after the metal (like NaHSO₄, NaHCO₃, KHSO₄). That marks an acid salt. A leftover OH marks a basic salt.