Equilibrium and Completeness of Reaction In class we are currently learning about equilibrium and Le Chateliers principal. The principal makes sense but seems contrary to some other concepts. For example, using stoichiometry to calculate the mass of reactants for a certain amount of products. If we know the reaction is always going to equilibrium where the forward and reverse reactions are happening at the same rate, then how will the reaction ever be "complete" because according to the principal even if the reaction were to be complete and all the reactants were gone, the reverse reaction should be occurring. This is not the case in reactions like burning magnesium.

I'm guessing that not all reactions are reversible and that the principal is correct but in some cases it would take a long time to reach equilibrium.
The basic stoichiometry is done to understand the basic concepts of moles and ratios in reactions. Later on, as the reactions become more complex, alot of them don't go to completion and so for a lot of them, you need equilibrium constants. This being said, not all reactions end partway, some of them do go to completion. The most basic example is disassociation of salt ions in solutions. From basic stoichiometry, you learn that the ratio is one to one (like NaCl -> Na+ + Cl-). So Moles of NaCl added = the moles of Na and Cl ions in water. So basic stoichiometry is sometimes (not always) applicable but is essential for the learning process.