Find the mass and number of molecules for: a) 44.8 liters of O₂ (oxygen) b) 67.2 mm H₂ (hydrogen)

44.8, this is 44.8 / 22.4 = 2 mol of oxygen, the mass of which is 2 * 32 = 64 g, the amount molecules = 2 * 6.02 * 10 ^ 23 = 1.2 * 10 ^ 24 molecules.
67.2 ml of H2, this is 0.0672 / 22.4 = 0.003 mol of hydrogen, the mass of which is 0.003 * 2 = 0.006 g = 6 mg. The number of molecules is 0.003 * 6 * 10 ^ 23 = 1.8 * 10 ^ 21 molecules.