How many lters of oxygen are needed to make 240 L of steam?

Question

2H2 O2 ==> 2H2O

Answer 1

since you are talking about steam from the above equation you will note oxygen is a diatomic molecule.  as such when reacting with hydrogen you will need 240L hydrogen and 120L oxygen for a balanced equation.

Answer 2

2H2 + O2 -> 2H20

since all are gases , so let all of them react in proprtion of their volume

for 1 l H20 -> 0.5  l O2 required

so for 240 l -> 120 l )2 required