JAMB Chemistry 2019 Paper

Concept:A chemical formula represents a pure substance with a fixed composition; mixtures and alloys have variable composition and cannot be given one formula.Explanation:Air is a mixture of gases like nitrogen, oxygen, and others, so it has no fixed chemical formula.Bronze is an alloy, a mixture of copper and tin, so it also has no single chemical formula.Sodium chloride ($\mathrm{NaCl}$), copper ($\mathrm{Cu}$), caustic soda ($\mathrm{NaOH}$), and washing soda ($\mathrm{Na}_2\mathrm{CO}_3$) are pure substances with definite formulas.Thus, the pair that cannot be represented with a chemical formula is air and bronze.Answer:A. air and bronze