when we do sto geometric calculations to figure out how much of a particular product to expect in a chemical reaction we are actually calculating the theoretical yield of that product this is a calculation that assumes that every molecule of reactant is flawlessly converted into products and it therefore represents the maximum amount of product that is physically possible but in actuality when we do chemistry in the laboratory it will never go so perfectly that every single molecule reacts the way we want them to we will always get some fraction of the theoretical yield which we call the percent yield the percent yield is defined as the actual yield over the theoretical yield time 100 in other words the percentage of the theoretical yield that is represented by what we actually get for example let's say we are performing The Following reaction where copper sulfate and zinc metal generate copper metal and zinc sulfate if we react 1.2745 N2 G of copper metal what was the percent yield for this reaction the first thing we must do is use stochiometry to calculate the theoretical yield converting this mass of copper sulfate into moles then using the stochiometric ratio in this case simply 1: one to get the moles of copper metal we would expect and then using the molar mass of copper to convert into a mass of copper we can see that we we should expect a theoretical yield of 0.507 G of copper metal again this is the maximum amount of copper that is physically possible if every single molecule of reactant was converted to products our actual yield or the mass that we physically measured at the end of the reaction was 0.392 G let's take our actual yield over our theoretical Yi yield times 100 and we can see that our percent yield was 77.3% in chemistry the goal is to get as close to 100% as possible since we want to maximize efficiency so our 77.3% isn't too bad but it leaves some room for improvement