WAEC Chemistry 2016 Paper

Concept:In a balanced ionic equation, the total charge on the left must equal the total charge on the right. Use this to find the number of electrons, $n$.Explanation:Write the total charge on the left-hand side.The charge from $\mathrm{Cr_2O_7^{2-}}$ is $-2$.There are $14$ hydrogen ions, each with charge $+1$, giving $+14$.Add the $n$ electrons, each with charge $-1$, giving $-n$.So total left charge $= -2 + 14 - n = 12 - n$.The right-hand side has $2$ chromium ions, each with charge $+3$.Total right charge $= 2 \times (+3) = +6$.Water molecules are neutral, so they contribute zero charge.For the equation to be balanced:$12 - n = 6$.Solve for $n$:$n = 12 - 6 = 6$.Answer:$n = 6$, which is option B.