Gases and the Mole Gay-Lussac formulated the law of combining volumes. Law of Combining Volumes Gases at the same temperature and pressure react with one another in volume ratios of small whole numbers. Law of Combining Volumes Example 1: H2 + Cl2 2HCl 1 L + 1 L 2 L Law of Combining Volumes Example 2: 2H2 + O2 2H2O 2 L + 1 L 2 L Avogadro?s law The volume of a gas, maintained at a constant temperature and pressure, is directly proportional to the number of moles of the gas. Molar Volume the volume that a mole of gas occupies at standard temperature and pressure Molar Volume 1 mole of ANY gas at STP occupies 22.4 L. Sample Problem 1 What volume would 7 moles of carbon dioxide occupy at STP? = 157 L CO2 7 mol CO2 22.4 L 1 mol Change 228 L of O2 at STP to moles. 470 moles 426 moles