H2 and O2 react according to the equation
2H2 (g) + O2 (g) ----------> 2H2O (g)
Thus, 2 volumes of H2 react with 1 volume of O2 to produce 2 volumes of water vapor. Hence, 10 volumes of H2 will react completely with 5 volumes of O2 to produce 10 volumes of water vapour.
