If 12kg of hydrogen is passed through s fuel cell, how much oxygen would be needed to react with it and how much water would be produced?

Molar mass of H₂ = 1 × 2 g/mol = 2 g/mol
Molar mass of O₂ = 16 × 2 g/mol = 32 g/mol
Molar mass of H₂O = 1×2 + 16 = 18 g/mol

Overall equation of the fuel cell : 2H₂ + O₂ → 2H₂O
OR: Mole ratio H₂ : O₂ : H₂O = 2 : 1 : 2

No. of moles of H₂ passed = (12000 g) / (2 g/mol) = 6000 mol

No. of moles of O₂ needed = (6000 mol) × (1/2) = 3000 mol
Mass of O₂ needed = (32 g/mol) × (3000 mol) = 96000 g = 96 kg

No. of moles of H₂O produced = (6000 mol) × (2/2) = 6000 mol
Mass of H₂O produced = (18 g/mol) × (6000 mol) = 108000 g = 108 kg

2H2 + O2 = 2H2O
moles (H2) = 12000g / 2 = 6000 moles
moles (O2) = 6000 / 2 = 3000
mass (O2)= 3000 x 32
mass(O2) = 96000 g = 96 kg.
Mass (H2O) = 12 kg + 96 kg = 108 kg. (Law of conservation of mass).