Determine the volume of H gas needed to react completely with 5.00 L of O to form water ?

Answer 1

2H2 + O2 ==> 2H2O

IF this is at STP for all components, you may dispense with converting to moles, then back to liters. The moles are proportional to volume.
Thus 5.00 L oxygen x (2 liters H/1 liter O) = ?? L H needed.