What is number of moles of oxygen gas needed to react with 4.0 moles of Mg?

The balanced chemical equation for the reaction between magnesium (Mg) and oxygen (O2) is:

2Mg + O2 -> 2MgO

From the balanced equation, it is clear that 1 mole of Mg reacts with 1/2 mole of O2. Therefore, if 4.0 moles of Mg reacts, you would need 2.0 moles of O2.