Iron (II) sulphide is heated in air to form compound A, and an oxide of sulphur. The oxide of sulphur is dissolved in water to give an acid. The basicity of this acid is ___.

Correct Answer: 2

Solution

Roasting gives $$4\text{FeS} + 7\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3 + 4\text{SO}_2$$, so A is $$\text{Fe}_2\text{O}_3$$. $$\text{SO}_2$$ dissolves in water to form sulphurous acid, $$\text{H}_2\text{SO}_3$$, which has two replaceable H atoms. Its basicity is 2.

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