Stoichiometry calculation for a chemical reaction.

The balanced equation is $2CO(g) + O_2(g) \rightarrow 2CO_2(g)$. With 4 moles of CO and 8 moles of $O_2$, CO is the limiting reactant. The reaction produces $4 \text{ moles CO} \times \frac{2 \text{ moles } CO_2}{2 \text{ moles CO}} = 8 \text{ moles } CO_2$. There will be $8 \text{ moles } O_2 - (4 \text{ moles CO} \times \frac{1 \text{ mole } O_2}{2 \text{ moles CO}}) = 4$ moles of $O_2$ left over.