Answer:
4.0 moles
Step-by-step explanation:
Based on the given reaction between carbon and hydrogen, the equation of the reaction between O₂ and CO is as follows:
2CO(g) + O₂(g) → 2CO₂(g)
The number of moles of O₂ needed to completely react with 8.0 moles of CO can be calculated knowing that 2 moles of CO react with 1 mol of O₂:

Therefore, the number of moles of O₂ needed is 4.0.
I hope it helps you!