JAMB Chemistry 2015 Paper

Concept:The oxidation state of an element in a compound is found by equating the total sum of all oxidation states to the overall charge, which is zero for a neutral compound.Explanation:In $\mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7$, potassium has an oxidation state of $+1$ and oxygen has an oxidation state of $-2$.Let the oxidation state of chromium be $x$.Write the sum of oxidation states: $2(+1) + 2x + 7(-2) = 0$.Simplify: $2 + 2x - 14 = 0$.This gives $2x - 12 = 0$, so $2x = 12$.Therefore, $x = +6$.Answer:The oxidation state of Cr in $\mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7$ is $+6$, which corresponds to option B.