the balanced equation for the above reaction is;
2H₂(g) + O₂ --> 2H₂O
Stoichiometry of H₂ to O₂ is 2:1
This means that the molar ratio in which H₂ reacts with O₂ is in the 2:1 ratio.
At constant temperature and pressure , number of moles are directly proportional to volume of gas. Therefore volume ratio in which H₂ reacts with O₂ is 2:1.
volume of 2 mL of H₂ reacts with 1 mL of O₂
therefore 250 mL of H₂ reacts with - 1/2 x 250 = 125 mL
volume of 125 mL of O₂ is required for the reaction