In a chemical reaction, the relative amounts of reactants used up and products formed are determined by the relative formula weights of the species and by their reaction coefficients. In the formation of water:
2H₂ + O₂ -> 2H₂O
2 moles of hydrogen react with 1 mole of oxygen to form 2 moles of water. To determine the mass amount of water formed from a given mass of oxygen:

1. Determine the chemical amount of oxygen initially present by dividing the mass of oxygen by the molecular weight of oxygen.
2. Since the ratio of the chemical amount of water formed to the chemical amount of oxygen used is 2:1, the chemical amount of water formed is two times the chemic (the ratio of their coefficients in the balanced equation)al amount of oxygen used.
3. The mass amount of water formed is determined by multiplying the chemical amount of water formed by the chemical amount of oxygen used.