Air is 20% oxygen.
Calculate the volume of air needed to provide enough oxygen to react with 3.50 dm³ of hydrogen gas.
The equation for the reaction is:
2H₂ + O₂ → 2H₂O

This problem involves using molar ratios of gases and percentage composition. **Part 1: Molar Ratio** The balanced equation is 2H₂ + O₂ → 2H₂O. According to Avogadro's law, for gases at the same temperature and pressure, the ratio of volumes is the same as the ratio of moles. The molar ratio of H₂ : O₂ is 2 : 1. This means that for every 2 volumes of hydrogen, 1 volume of oxygen is required. So, the volume of oxygen needed is half the volume of hydrogen. Volume of O₂ = 3.50 dm³ / 2 = 1.75 dm³. **Part 2: Percentage Composition** We need 1.75 dm³ of pure oxygen. We are told that air is 20% oxygen by volume. Let V_air be the total volume of air. Then, the volume of oxygen is 20% of V_air. Volume of O₂ = 0.20 × V_air We can rearrange this to find the volume of air: V_air = Volume of O₂ / 0.20 V_air = 1.75 / 0.20 V_air = 8.75 dm³. Alternatively, since 20% is 1/5, the volume of air is 5 times the volume of oxygen: V_air = 1.75 × 5 = 8.75 dm³.