If 10.4 L of H2 reacts with 5.2 L of O2 at STP, what is the volume of the gaseous water collected (assuming that none of it condenses)?

Answer 1

You have twice as many mols of H2 as of O2
2 H2 + O2 ---> 2 H2O
So in fact you needed exactly that many
You got one liter of gas for every liter of H2
V = 10.4 liters