Question

How many moles of O2 are consumed if 35 moles of octane are burned?

Answer 1

According to the reaction, 2 molecules of octane react with 25 molecules of oxygen. Therefore, 35 moles of octane correspond to 2 molecules, and x moles of oxygen correspond to 25 molecules. Thus:

$$x=35\cdot25/2=437.5\text{ moles}.$$