The product of a chemical reaction is calculated in two forms; theoretical yield and actual yield. Actual yield of a chemical reaction is the actual amount of product obtained when a chemical reaction takes place and theoretical yield is the maximum amount of yield that can be obtained from a chemical reaction. Theoretical yield can be determined through calculations while to calculate actual yield we need to perform the chemical reaction. This article on how to calculate theoretical yield will help you in calculating theoretical yield.
Ways to calculate theoretical yield
- To calculate the theoretical yield, the reaction between the limiting reagent and the product is multiplied by the number of moles of the limiting reagent used in a chemical reaction.
- Initially, balance the chemical reaction and calculate the number of moles of each starting material used.
- Now, find the limiting reagent. Limiting reactants or limiting reagents are those reactants which react completely and not act as a supporting reactant or a catalyst for the chemical reaction.
- Calculate the moles of the product expected and then calculate grams of product corresponding to the number of moles expected.
Identifying limiting reagent while calculating theoretical yield
- If you want to determine the limiting reagent for solids, then divide the mass of the solid reactant taken in grams by its molecular weight in g/mol.
- If you want to determine the limiting reagent for liquids and gases, then multiply the volume by density and then divide by molecular weight.
- Now, to find out the limiting reagent multiply the molecular weight by the number of moles used in the reaction. The reactant which has the lowest number is the limiting reagent.
Example for calculating theoretical yield
- Consider the reaction in which 4 moles of Al react with 3 moles of O2 to form 2 moles of Al2O3 when the reaction is balanced.
4Al + 3O2 ————–> 2Al2O3
- In the above reaction Al is the limiting reagent, so divide the number of Al moles used by 2. This is because it takes 4 moles of Al to make 2 moles of Al2O3. The ratio obtained from this chemical reaction would be 2:1.
- Now, you need to multiply the number of moles of the product by the molecular weight of the product.
- Molecular weight of Al2O3 is 101.96 g/mol. If the number of moles of product obtained from the reaction is 0.5 moles, then multiplying it with the molecular weight of Al2O3 you will get 50.98 grams. This is the theoretical yield of the above chemical reaction.
- This same procedure is followed to calculate theoretical yield of solids, liquids and gaseous reactions.
- We basically calculate theoretical yield to know the efficiency of a chemical reaction. When the theoretical yield of a chemical reaction is compared with the actual yield we will know efficiency of a chemical reaction. Efficiency of a chemical reaction is the ratio of actual yield to the theoretical yield and is expressed in percentage.
Yield (in %) = (actual yield (grams) / theoretical yield (grams)) x 100
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