The balanced reaction equation for combustion of Mg is: 2 Mg + O2 --> 2 MgO.

Question

In the reaction vessel you have 1.0 moles of Mg and 0.30 moles of O2. What is the limiting reactant? How many moles of MgO can you possibley make?

Answer 1

Here is a worked example of a simple stoichiometry problem. Follow the steps.
http://www.jiskha.com/science/chemistry/stoichiometry.html
Use the procedure twice, once for one reactant and next for the other reactant. This will give you two answers for moles CO2; the smaller one is the correct value to choose and the reactant giving that value is the limiting reagent.