If a current plates out 13.5 g of aluminium, what mass of magnesium would be plated out in the same time by the same current?

Molar mass of Al = 27.0 g/mol
Molar mass of Mg = 24.3 g/mol

Consider the electroplating of aluminium:
Al³⁺ + 3e⁻ → Al
Mole ratio e⁻ : Al = 3 : 1
Moles of Al plated = (13.5 g) / (27.0 g/mol) = 0.5 mol
Moles of e⁻ passed = (0.5 mol) × 3 = 1.5 mol

Consider the electroplating of magnesium:
Mg²⁺ + 2e⁻ → Mg
Mole ratio e⁻ : Mg = 2 : 1
Moles of e⁻ passed = 1.5 mol
Moles of Mg plated = (1.5 mol) × (1/2) = 0.75 mol
Mass of Mg plated = (0.75 mol) × (24.3 g/mol) = 18.2 g

Answer: 18.2 g Mg