If 10 volume of dihydrogen gas reacts with 5 volume of dioxygen gas, how many volumes of water vapour would be produced ?

$2{\text{H}}_2+{\text{O}}_2\underset{ }{\overset{ }{\to }}2{\text{H}}_{\text{2}}\text{O}$

Volume of gas is directly proportional to the number of moles. Given reaction indicates that 2 vol of H₂ react with 1 vol of O₂ to form 2 vol of water. 

Hence, 10 volumes of dihydrogen react with 5 volume of oxygen to form 10 volumes of water.