To calculate theoretical mass, or theoretical yield, one must balance the reaction, establish the number of moles, find the reagent that is limiting and then calculate the moles and grams of the product expected to be yielded. These theoretical calculations are actually done the same way as general chemistry equations in that how many moles are in the reactant product will determine the number of moles in the product.
The reaction must be balanced so the ratio of reactants to products can be found, according to an organic chemistry guide provided by University of Colorado at Boulder. Once this is determined, each starting material must have its designated number of moles.
The limiting reagent has to be determined, and it's important to note that this cannot be anything that is not a part of the actual chemical reaction. The assumption should be made that 100 percent yield will occur based on the reagent, in theory.
Another calculation determines the grams of product expected from the number of moles expected from the reaction. This is how to calculate theoretical mass, in grams, also referred to as theoretical yield, but percent yield can also be easily determined once the theoretical yield has been calculated by simply dividing the actual yield in grams by the theoretical yield and then converting the figure into a percentage.