Hydrogen gas and oxygen gas combine to form water. How many grams of oxygen are needed to react with 45.7 grams of hydrogen?

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