Theoretical yields are the amount of products that are supposed to be created by a chemical reaction if none of the reactants were wasted and the reaction was fully completed. Knowing the theoretical yield helps determine how efficiently a reaction was carried out, which can be important in industrial settings for maximizing profitability.
Determine the limiting reagent. For solids, divide the mass of a reactant used by its molecular weight. For liquids and gases, multiply the volume times the density and then divide by the molecular weight.
Multiply the molecular weight by the number of moles in the equation. Whichever reactant has the smallest number is the limiting reagent.
Calculate the theoretical mole yield by using the chemical equation. The ratio between the limiting reagent and the product is multiplied by the number of moles of the limiting reagent used in the experiment. For example, if your equation was 4Al + 3O2 yields 2 Al2O3, and Al was your limiting reagent, you would divide the number of Al moles used by two because it takes four moles of Al to make two moles of Al2O3, a ratio of two to one.
Multiply the number of moles of the product by the molecular weight of the product to determine the theoretical yield. For example, if you created 0.5 moles of Al2O3, the molecular weight of Al2CO3 is 101.96 g/mol, so you would get 50.98 grams as the theoretical yield.
Make sure you use consistent units; don't use interchange English and standard units.