BECE Science 2025 Paper

Concept:Iron(II) sulphide contains $\mathrm{Fe}^{2+}$ and $\mathrm{S}^{2-}$ ions, which combine in a 1:1 ratio.Solution:Iron(II) means iron has a charge of $+2$, written as $\mathrm{Fe}^{2+}$.Sulphide ion has a charge of $-2$, written as $\mathrm{S}^{2-}$.Since the charges are equal and opposite, one $\mathrm{Fe}^{2+}$ combines with one $\mathrm{S}^{2-}$.Thus, the formula is $\mathrm{FeS}$.Key point: $\text{FeS}_2$ is iron pyrite (disulphide) and $\text{Fe}_2\text{S}_3$ is iron(III) sulphide; the Roman numeral (II) fixes the charge of iron at $+2$.Answer:Option C ($\text{FeS}$)