The limiting reactant is the chemical substance that determines the amount of product(s) that can ultimately be formed in a reaction. during the reaction, the limiting reactant is completely consumed or used up, and therefore, causes the reaction to stop. the limiting reactant can be identified through stoichiometric calculations. after comparing the results, the reactant that produces the smaller mass of product is identified as the limiting reactant. determine the limiting reactant and calculate the number of grams of al2o3 that can be formed when 10.0 grams of al and 5.00 grams of o2 react. 4 al(s) + 3 o2(g) → 2 al2o3(s) for the calculations in this module, the molar mass of an element will be rounded to the hundredths place (0.01 g).