Gas Volume Calculation - 2

Question: 50 cm³ of propane is burned completely in 300 cm³ of oxygen. All volumes are measured at room temperature and pressure. Find the volume of oxygen used, the volume of carbon dioxide formed and the total volume of gas left at the end.C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l)By Avogadro's law, equal volumes of gases at the same temperature and pressure contain the same number of molecules. So for gases, the ratio of volumes is the same as the ratio of moles in the equation. There is no need to convert to moles.Step 1: oxygen used. The ratio C₃H₈ : O₂ is 1 : 5.50 × 5 = 250 cm³ of O₂Step 2: carbon dioxide formed. The ratio C₃H₈ : CO₂ is 1 : 3.50 × 3 = 150 cm³ of CO₂Step 3: oxygen left over.300 - 250 = 50 cm³ of unreacted O₂Step 4: total gas volume. At rtp water is a liquid, so its volume is negligible and it is not counted.150 + 50 = 200 cm³Answer: 250 cm³ of oxygen is used, 150 cm³ of carbon dioxide forms and 200 cm³ of gas remains.This shortcut only works for substances that are gases under the conditions stated. Solids and liquids must be dealt with using moles and masses.