JAMB Chemistry 2019 Paper

Concept:Oxidation numbers are assigned using known values for other atoms and the overall charge on the ion.Explanation:In this redox reaction, chromium changes from $\mathrm{Cr}_2\mathrm{O}_7^{2-}$ to $\mathrm{Cr}^{3+}$.Let the oxidation number of each Cr atom in $\mathrm{Cr}_2\mathrm{O}_7^{2-}$ be $x$.Oxygen has oxidation number $-2$, and the total charge of the ion is $-2$.So: $2x + 7(-2) = -2$This gives $2x - 14 = -2$Therefore $2x = 12$, so $x = +6$.On the product side, chromium appears as $\mathrm{Cr}^{3+}$, so its oxidation number is $+3$.Thus, the oxidation number of chromium changes from $+6$ to $+3$.Answer:$+6$ to $+3$ (Option B).