JAMB Chemistry 2024 Paper

Concept:An ionic compound is correct only when the total positive charge equals the total negative charge.Explanation:Check the formula of each compound by using the charges on the ions involved.In Aluminiumtetraoxosulphate(VI), $\mathrm{Al}^{3+}$ combines with $\mathrm{SO}_4^{2-}$, so the formula is $\mathrm{Al}_2(\mathrm{SO}_4)_3$. This is correct.In Calciumtrioxonitrate(V), $\mathrm{Ca}^{2+}$ combines with $\mathrm{NO}_3^{-}$, so the formula is $\mathrm{Ca}(\mathrm{NO}_3)_2$. This is correct.In Iron(III)bromide, $\mathrm{Fe}^{3+}$ combines with $\mathrm{Br}^{-}$.One $\mathrm{Fe}^{3+}$ ion needs three $\mathrm{Br}^{-}$ ions to balance the charges.Hence, the correct formula is $\mathrm{FeBr}_3$ and not $\mathrm{Fe}_3\mathrm{Br}$.In Potassiumsulphide, $\mathrm{K}^{+}$ combines with $\mathrm{S}^{2-}$, so the formula is $\mathrm{K}_2\mathrm{S}$. This is correct.Answer:The compound whose formula is not correct is Iron(III)bromide, given as $\mathrm{Fe}_3\mathrm{Br}$, which is option C.