From the reaction stoichiometry we find that 25/2 moles of oxygen are required. Next, we calculate the volume using the ideal gas law, which is 308 liters-- now you know why a crowded highway has a significant effect on our atmosphere.
Finally, we will determine the amount of air consumed. Since air is 21% oxygen, there are 100 moles of gas molecules of all types for each 21 moles of oxygen in the atmosphere. From the discussion on gas mixtures, you know that 100 moles of gas occupies 100 over 21 times as much volume as 21 moles of gas. Therefore, the volume of air used to burn one mole of octane is 100 over 21 times as large as the volume of oxygen used in the reaction.