Question

If 30ml of H2 and 20ml of O2 reacts to form water, what is left at the end of the reaction?

Answer 1

2 H2 + O2 → 2 H2O 

20 ml of O2 would react completely with 20 x (2/1) = 40 ml of H2, but there is not that much H2 present, so H2 is the limiting reactant, and O2 is in excess. So at the end of the reaction there is water vapor and left-over O2.