Homework for Tuesday:  48-52 p. 392
Also I would like to work # 47 again so I am sure you understand.  I am going to take a little different approach.

                  25.0L    X= 312.5L
             2 C₈H₁₈  +  25 O₂  -->  16 CO₂  +  18 H₂O
                 2 mol        25 mol
I just used a volume to mole ratio.
Now if air is 21% O₂, than 312.5L is only 21% of what you need.
So,          21%   =   100%
               312.5L       X
                       X=  1488L or 1500L using two sig fig's
Make sure you understand what the ideal gas law constant is and how it got its' value.