The balanced equation for water is 2 H2 + O2 to 2 H2O. If I have 21.2g of a product, and I started with 5.6 g of H2, how many grams of oxygen were reacted

Since 21.2 g H2O was produced, the amount of oxygen that reacted can be obtained using stoichiometry. The balanced equation was given: 2H₂ + O₂ → 2H₂O and the molar masses of the relevant species are also listed below. Thus, the following equation is used to determine the amount of oxygen consumed.

Molar mass of H2O = 18 g/mol

Molar mass of O2 = 32 g/mol

21.2 g H20 x 1 mol H2O / 18 g H2O x 1 mol O2 / 2 mol H2O x 32 g O2 / 1 mol O2 = 18.8444 g O2

We then determine that 18.84 g of O2 reacted to form 21.2 g H2O based on stoichiometry. It is important to note that we do not need to consider the amount of H2 since we can derive the amount of O2 from the product. Additionally, the amount of H2 is in excess in the reaction.