For the complete combustion of 40.0 liters of hydrogen gas (H_2) at STP, what volume of oxygen gas (O_2) is required?

Since the reaction occurs at STP (Standard Temperature and Pressure), use the fact that volumes of gases at STP are directly proportional to moles, so the volume ratio is the same as the mole ratio.

Set up the volume ratio based on the balanced equation: for every 2 volumes of H\_2, 1 volume of O\_2 is required.