WAEC Chemistry 2011 Paper

Concept:In a balanced redox equation, the total charge on the reactant side must equal the total charge on the product side.Explanation:Find the total charge on the left side.$\mathrm{Cr_2O_7}^{2-}$ contributes $-2$.$14\mathrm{H}^{+}$ contributes $+14$.$n\mathrm{e}^{-}$ contributes $-n$.Total left charge $= -2 + 14 - n = 12 - n$.Find the total charge on the right side.$2\mathrm{Cr}^{3+}$ contributes $2 \times (+3) = +6$.$\mathrm{H_2O}$ is neutral, so it contributes $0$.Total right charge $= +6$.Equate the charges on both sides.$12 - n = 6$$n = 12 - 6$$n = 6$Answer:$n = 6$