Theoretical yield is a term in chemistry that refers to the amount of product you would have after a chemical reaction if that reaction went to completion. For a reaction to go to completion all of the limiting reactant must be used, making it impossible for more product to be formed from what remains. To find the theoretical yield, you must know the equation for the reaction and how many moles of each reactant you are starting with.
Balance the chemical equation. For example, take the equation H + O = H2O. To balance this you need two hydrogen on the left to balance the two hydrogen in water, so 2H + O = H2O.
Determine the limiting agent. This is the agent you will run out of first in the reaction. For the example, assume that you start with 5 moles of hydrogen and 3 moles of oxygen. You need a 2:1 ratio of hydrogen to oxygen, as can be seen in the equation. To use 3 moles of oxygen would require 6 moles of hydrogen (3 moles x 2) but you only have 5. Therefore, hydrogen is the limiting agent in this example.
Calculate the resulting moles of product based on the amount of the limiting agent. Do this by multiplying the moles of the limiting agent by the ratio between the product and the limiting agent. In the example, the ratio between H2O and hydrogen is 1:2. So, 1/2 x 5 moles H = 2.5 moles of H2O. This is the theoretical yield.
TL;DR (Too Long; Didn't Read)
Convert the theoretical yield from moles to grams using the molar weight of the product.