What would be the result if 5.0 L of hydrogen were burned with 2.0 L of oxygen in a closed containner?

Answer 1

2H2 + O2 ==> 2H2O
First determine the limiting reagent. (When BOTH reactants are given you know it is a limiting reagent problem.)
5 L H2 x (2 moles H2O/2 moles H2) = 5 L x (2/2) = 5 L H2O.
2 L O2 x (2 moles H2O/1 mole O2) = 4 L H2O.

Both answers can be right; the correct answer in limiting reagent problems is ALWAYS the smaller value.
Some of the H2 will remain un-reacted.