Q.If 10 volumes of dihydrogen gas react with 5 volumes of dioxygen gas, how many volumes of water vapour would be produced?

Step 1. From the reaction 2H2(g) + O2(g) → 2H2O(g), hydrogen, oxygen and water vapour react/form in the fixed volume ratio 2 : 1 : 2 (Gay-Lussac's law of gaseous volumes).

Step 2. The given amounts, 10 volumes of H2 and 5 volumes of O2, are already in exactly the required 2:1 ratio (10:5 = 2:1), so both gases are used up completely with nothing left over. …

Apply Gay-Lussac's 2:1:2 volume ratio for the hydrogen-oxygen-water reacti …

• Simply adding the two given volumes (10+5=15) instead of applying the correct 2:1:2 combining ratio. …