Calculating Theoretical Yield

Theoretical yield can be defined as the amount of product formed on completion of the reaction, when there is no wastage of the reactants. Computing theoretical yield in chemistry depends upon the ideal conditions, which cause complete or total consumption of the reactants and no other losses take place and stoichiometry of the reaction. Under ideal conditions, reversible reaction also does not take place. Ideal conditions also obstruct side reactions, so that there is no loss of ingredients.

Stoichiometry, one of the chemistry terms, is defined as the calculation of products formed and reactants consumed during the reaction, or in simple words, you can call it as a quantitative relationship between products and reactants. While calculating it in chemistry, first thing that needs to be done is, balancing the reaction. The yield of the reaction is based on the limiting reagent, also termed as limiting reactant, which determines the extent of the reaction before the reaction is complete. The ingredient which is present in less number of moles than required is the limiting reagent for that particular reaction.

After balancing the reaction, find out which reactants are limiting reactants and which are excess. It should be noted that any ingredient such as solvent or for instance, catalyst is not a part of limiting reagent. The moles of the product formed, or the yield of the reaction depends upon the moles of limiting reactant.

Theoretical Yield Calculation Example

Let's go through a simple scenario, wherein, we will try calculating theoretical yield of Aspirin. Aspirin is prepared from salicylic acid. 1 mole of aspirin is obtained from 1 mole of salicylic acid. Suppose, we consider a scenario, where in an experiment, we got 121.2 grams Aspirin from salicylic acid, 100 grams. We will calculate percent yield of Aspirin. C9H8O4 is the chemical formula for Aspirin and C7H6O3 is the chemical formula for salicylic acid. Example is given below:

Step 1: Calculating relative molecular mass from the relative atomic mass. We know that atomic mass of oxygen is 16, that of hydrogen is 1 and atomic mass of carbon is 12. from this we can calculate molecular mas of Aspirin as [(12*9)+(1*8)+(16*4)], which comes out to be 180. Similarly, for salicylic acid, molecular mass can be calculated as [(12*7)+(1*6)+(16*3)], which comes out to be 138.

Step 2: We know that 1 mole of salicylic acid is = 138 Grams. So, 100 grams = (100/138) which equals 0.725 moles.

Step 3: As we know, 1 mole of Aspirin is obtained from 1 mole of salicylic acid, therefore, 0.725 moles of Aspirin will be obtained from 0.725 moles of salicylic acid. (0.725*180) will be the calculated mass, which is equal to 130.5 grams.

Step 4: From the formula, % Yield = (Actual Yield/Theoretical yield)*100, we can calculate % yield for aspirin. (121.2 ÷ 130.5 × 100) gives us the value of percent yield to be 92.873.

Calculating theoretical yield leads to finding out the efficiency of the reaction, which is the key for maximum profits in chemical industries such as, cosmetics, pharmaceuticals or fertilizers. This is done to achieve maximum yield at minimum production cost. Low yield implies that the conditions during the reaction were not optimum and side reactions, which we talked about, may have taken place or there may be some losses during the reaction. Actual yield is defined as the amount of the product actually formed by the reaction. Theoretical yield is always more than the actual yield, since we assume ideal conditions.

After calculating % yield, you can further find % purity with the formula [% purity= (pure product mass / impure product mass)*100]. While calculating theoretical yield or % yield, follow the steps described above.