BECE Science 2023 Paper

Concept:In a chemical formula, the small number (subscript) written after a symbol gives the number of atoms of that element in one molecule. A symbol with no subscript means one atom.Solution:$\mathrm{H_2O_2}$ is hydrogen peroxide.$\mathrm{H_2}$: the subscript $2$ means $2$ hydrogen atoms.$\mathrm{O_2}$: the subscript $2$ means $2$ oxygen atoms.Total number of atoms in one molecule $= 2 + 2 = 4$.Key point: Do not confuse $\mathrm{H_2O_2}$ with water, $\mathrm{H_2O}$, which has $2 + 1 = 3$ atoms (option C).Answer:Option B (4)