Question

What volume will water vapor form when 40 liters of hydrogen completely reacts with 25 liters of oxygen? Assume the temperature and pressure remains constant.

Answer 1

                                                             2H2           +           O2 ---->2H2O  
number of moles in reaction                 2 mol                  1 mol       2 mol
number of liters in the reaction         2*22.4 L                1*22.4 L    2*22.4L
We can see that volumes of the gases are proportional to coefficients in the reaction ( if gases are under the same conditions), so we can write

                              2H2           +           O2 ---->2H2O  
                               2 L                         1 L          2 L
     given                 40 L                      ( 25 L)     40 L

We can see that we have excess of O2,
because if 2 L H2 are needed 1 L O2, then 40 L of H2 are needed 20 L O2.

So, limiting reactant is H2, and we will need to calculate Volume of H2O using H2.
2L H2 give 2L H2O(gas), so 40 L H2 give 40 L H2O.