Hydrogen peroxide is used to clean small wounds. It has a limited shelf life, however, because the following decomposition reaction occurs:
2 H₂O₂ (aq) → 2H₂O (l) + O₂ (g)

This is why you often hear a gas release when you open a bottle of old hydrogen peroxide. If 35 mL of a 1.20 M solution of hydrogen peroxide completely decomposes, how many grams of oxygen are produced?

                      0.672 g of O₂

Explanation:

                   The balance chemical equation for the decomposition of hydrogen peroxide is as follow;

                                   2 H₂O₂ → 2 H₂O + O₂

Step 1: Calculate Moles of H₂O₂ as;

                     Molarity  =  Moles / Volume of Solution

Solving for Moles,

                     Moles  =  Molarity × Volume of Solution

Putting values,

                     Moles  =  1.20 mol.L⁻¹ × 0.035 L         ∴ 1 mL  =  0.001 L

                     Moles  =  0.042 moles of H₂O₂

Step 2: Calculate Moles of O₂ as;

According to equation,

                        2 moles of H₂O₂ produced  =  1 mole of O₂

So,

                  0.042 moles of H₂O₂ will produce  =  X moles of O₂

Solving for X,

                     X =  0.042 mol × 1 mol / 2 mol

                     X =  0.021 moles of O₂

Step 3: Calculate Mass of O₂ as;

                     Mass  =  Moles × M.Mass

                     Mass  =  0.021 mol × 32 g/mol

                     Mass  = 0.672 g of O₂