Two moles of magnesium (Mg) and five moles of oxygen (O2) are placed in a

reaction vessel. When magnesium is ignited, it reacts with oxygen. What is the limiting reactant in this experiment?

Mg + O2 → MgO (unbalanced)

a. Mg b. O2 c. MgO d. both the reactants are in the same proportion

To shorten this the answer is A) Mg

First off, the chemical equation needs to be balanced. Since there are 2 oxygen atoms in the reactants, we must produce 2 moles of MgO, which means we need 2 moles of Mg, giving:

2Mg + O2 - - > 2MgO

Therefore for a stoichiometric reaction where no reactant is left over, the number of moles of magnesium must be double that of oxygen. However, as seen, the molar ratio of magnesium to oxygen is 2:5, meaning oxygen is in large excess, and Magnesium is the limiting reactant. Therefore the answer is A.