WAEC Chemistry 2019 Paper

Concept:The sum of oxidation numbers of all atoms in a polyatomic ion equals its overall charge.Explanation:Let the oxidation number of chromium be $x$.In $\mathrm{Cr}_2\mathrm{O}_7^{2-}$, there are 2 chromium atoms and 7 oxygen atoms.Oxygen has an oxidation number of $-2$.So, total oxidation contribution of oxygen is $7(-2) = -14$.Total charge of the ion is $-2$.Therefore, $2x + (-14) = -2$.$2x - 14 = -2$$2x = -2 + 14$$2x = 12$$x = +6$Answer:The oxidation number of chromium in $\mathrm{Cr}_2\mathrm{O}_7^{2-}$ is $+6$, which is option C.