The balanced equation:
2 H₂ + O₂ → 2 H₂O
moles of Hydrogen = mass of hydrogen ÷ molar mass of hydrogen
= 45.7 g ÷ 2 g/mol
= 22.85 mol
Now, the mole ratio of O₂ : H₂ is 1 : 2
which means that there are two times as much moles of H₂ as O₂ taking
part in the reaction
Thus since moles of H₂ = 22.85 mol
then moles of O₂ = 22.85 ÷ 2 = 11.425 mol
Mass of O₂ = moles of O₂ × molar mass of O₂
= 11.425 mol × 32 g/mol
= 365.6 g
∴ grams of oxygen are needed to react with 45.7 grams of hydrogen is 365.6g