1. Write the balanced chemical equation:

2 Mg + O₂ → 2 MgO

2. Determine the mole ratio:

From the balanced equation, we see that 1 mole of O₂ reacts to produce 2 moles of MgO.

3. Calculate the moles of MgO produced:

Since 0.200 mol of O₂ reacts completely, we can use the mole ratio to find the moles of MgO produced:

0.200 mol O₂ * (2 mol MgO / 1 mol O₂) = 0.400 mol MgO

Therefore, 0.400 moles of MgO are produced when 0.200 mol of O₂ reacts completely with magnesium.