Write balance reaction for electrolysis of water and use Faraday's law and mole concept. 2H2O ⇌ 4H++2O2− At cathode, 4H++4e− → 2H2 At anode, 2O2−−4e− → O2 ∴ 4 faraday of charge liberates = 1 mol = 22.4 dm3O2 ∴ 1 faraday of charge liberates =