Question

How many moles of Co2 will be produced if 9.0 moles of O2 completely react?

Answer 1

 It depends on what was being combusted. 

Supposing the simplest case, where carbon is being burnt: 

C + O2 → CO2 

(9.0 mol O2) x (1 mol CO2 / 1 mol O2) = 9.0 mol CO2 

The answer would be different if the fuel contained any oxygen atoms, for example, alcohol.