Iron sulphide is heated in air to form A, an oxide of sulphur. A is dissolved in water to give an acid. The basicity of this acid is

When iron sulphide (FeS) is heated in air, it reacts with oxygen to form iron oxide (FeO) and sulphur dioxide (SO₂):

4FeS + 7O₂ → 2Fe₂O₃ + 4SO₂

The oxide of sulfur (A) formed is sulfur dioxide (SO₂). When dissolved in water, sulfur dioxide reacts with water to form sulfurous acid (H₂SO₃):

SO₂ + H₂O → H₂SO₃

Sulfurous acid is a weak acid that has two acidic hydrogen atoms. The basicity of an acid is defined as the number of replaceable hydrogen atoms in one molecule of the acid. In the case of sulfurous acid, there are two acidic hydrogen atoms, so its basicity is 2.

Therefore, the acid formed when A is dissolved in water has a basicity of 2.