Which element is the excess reagent when 3.00 moles of Mg is ignited in 2.20 moles of pure oxygen in the reaction below?

2Mg + O2 -> 2MgO

Oxygen is in excess.

Explanation:

The coefficients of the balanced equation create a mole ratio that shows the ratio of how many reactants are used up and products are created.  

The mole ratio of Mg to O2 in this equation is 2:1, which means that for every two moles of Mg used, there will be 1 mole of O2 used.

If we have 3.00 moles of Mg, we will only need 1.5 moles of oxygen to completely burn the Mg. Therefore, when all 3.00 moles of Mg are used, there will still be some of the 2.20 moles of oxygen remaining.