Relative atomic mass of copper (Cu) = 64g/mol
(Based on O level periodic table)

Number of moles of Cu in 16g = 16g ÷ 64g/mol
= 0.25 mol

Next,

Cu²+ 2e → Cu

Each Cu²+ ion gained two electrons and was reduced to the Cu atom.

So for 0.25mol of Cu, 0.5mol of electrons were supplied (0.25 mol × 2 = 0.5 mol)


The same amount of charge (0.5 mol of electrons) deposited 70g of cerium.

Number of mol of cerium in 70g
= 70g ÷ 140g/mol
= 0.5 mol


Since 0.5 mol of electrons deposited 0.5 mol of cerium, the ratio is 1 : 1

i.e

Ce ≡ e

So each Ce atom was deposited after gaining 1 electron.

The equation would be :

Ce+ + e →Ce


Answer : (A)