Question

2Mg + O2 = 2MgO.Show that Mg is the limiting reactant?

Answer 1

If you are reacting in the atmosphere, then the oxygen is in excess (hopefully atmospheric oxygen will never be the limiting reagent) so since there is a limited supply of Mg that means that it is the limiting reagent

Answer 2

If this helps let me know

The limiting reagent is the one that will be all used up if the reaction goes to completion. There is not enough of it present to use up all the other reagent (which is in excess). :)