The balanced chemical reaction:
C3H8 (g) + 5 O2 (g) = 3 CO2(g) + 4 H2O (l)
To determine the amount of carbon dioxide, we use the relation of the substances from the reaction, the initial amount of the reactant O2. We do as follows:
2.5 mol O2 (3 mol CO2 / 5 mol O2) = 1.5 mol CO2 produced