To calculate the mass of 78 cm³ of oxygen at room temperature and pressure, the molar volume of gas is used, which is 24 liters. The number of moles is determined by dividing the volume by the molar volume, leading to 78/24000 moles of oxygen. Each mole of oxygen (O₂) has a molar mass of 32 g, as it consists of two oxygen atoms, each with a mass of 16 g. There is confusion regarding whether to use 32 or 16 for calculations, particularly in relation to the decomposition of hydrogen peroxide (H₂O₂), which produces half a mole of O₂. Ultimately, the correct mass calculation for 78 cm³ of oxygen is confirmed to be 0.104 g.