# In the reaction 2Mg + O_2 -> 2MgO, the law of definite proportions states that for every 2 moles of Mg you will need how many moles of O_2?

It's in the balanced equation. Each number before a compound corresponds to how many miles you use, and when there is no number you put in a 1. So 2 $M g$ + (1) ${O}_{2}$ means twice as many moles of $M g$ as ${O}_{2}$.