At stp 1 mole of a gas =22.4l
calculate the number of moles of CH4 formed
2.50/22.4=0.1116moles
from the reaction above the reacting ratio of H2 to CH4 is 4:1
therefore the moles of H2 reacted is 0.1116 x4 =0.4464moles
volume of H2 =0.4464 x 22.4 l =9.99l