Key Idea (i) H2+21O2⟶H2O (ii) Amount of water produced is decided by limited reactant (ie, the reactant which is used in small amount) H2+21O2⟶H2O1 mol21 mol1 mol210 mol3264 mol=5 mol=2 mol∵21molO2 gives =1 molH2O∴2 molO2 will give =1×2×2=4 mol.