JAMB Chemistry 2024 Paper

Concept:An acid anhydride is a non-metal oxide that reacts with water to form an acid.The form $\mathrm{XO}_2$ means the oxide contains one atom of element $X$ and two atoms of oxygen.Explanation:Sulphur burns in oxygen to form sulphur dioxide, $\mathrm{SO}_2$, which fits the general formula $\mathrm{XO}_2$.When $\mathrm{SO}_2$ reacts with water, it produces sulphurous acid:$$\mathrm{SO}_2 + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{H}_2\mathrm{SO}_3$$So $\mathrm{SO}_2$ is the acid anhydride of sulphurous acid.Phosphorus forms oxides such as $\mathrm{P}_4\mathrm{O}_{10}$ and $\mathrm{P}_2\mathrm{O}_5$, not a simple $\mathrm{XO}_2$ anhydride.Chlorine and fluorine do not form the required acidic oxide of the form $\mathrm{XO}_2$ in this context.Hence, sulphur is the element that gives an acid anhydride of the form $\mathrm{XO}_2$.Answer:D. Sulphur, because it forms sulphur dioxide, $\mathrm{SO}_2$, an acid anhydride fitting $\mathrm{XO}_2$.