Every chemical reaction has an ideal yield and a chemical yield. You would get your ideal, or theoretical, yield if your chemical process were 100 per cent efficient. Since no chemical reaction actually proceeds with 100 per cent efficiency, scientists use the concept of the chemical yield to describe the efficiency that a chemical reaction has in real life. Calculating chemical yield consists of two parts: You need to calculate the theoretical yield, and then compare the actual yield to the theoretical yield to get chemical yield.
- Skill level:
- Moderately Challenging
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Things you need
- Periodic table
Write down the chemical equation in question. The lab worksheet or your practice problem will probably give you this information.
Calculate the molar mass of each chemical that you start with. To find the molar mass, use a periodic table to add up the molecular weights of all of the elements in a compound. For example, the molar mass of sodium chloride (NaCl) is 58 grams, because one mole of sodium weighs 23 grams and a mole of chloride weighs 35 grams. You can calculate the molar mass of potassium iodide by adding 39 grams for potassium to 127 grams for iodine, for a total molecular mass of 166 grams.
Divide the molecular mass of each reagent by the number of grams that you start with to determine the number of moles of each reagent that you have. If you begin your reaction with 100 grams each of NaCl and KI, you have 100/58=1.72 moles of NaCl and 100/166=0.6 moles of KI.
Figure out which reagent is the limiting reagent, which is the chemical with the smallest number of moles. Because the limiting reagent runs out first, your reaction cannot proceed after it runs out. In this case, your limiting reagent is KI.
Set the number of moles of limiting reagent equal to the total maximum theoretical yield. Since you have 0.6 moles of KI, which is the limiting reagent, your theoretical yield is 0.6 moles.
Calculating the Theoretical Yield
Find the information in the problem about the quantity of product that is produced from the reaction. The problem will most likely give you this information in units of grams.
Convert your product units from grams to moles. You can do this as described above: add up the molecular masses of each element in the compound, and then divide the molecular mass by the number of grams that you have.
Divide the number of moles that you got in the last step by the theoretical yield that you calculated in the last section. If your actual yield is 0.4 moles, then your yield is 0.4/0.6=0.66.
Multiply the number that you got in the last section by 100 to get a final percentage chemical yield. In this case, 0.66 multiplies by 100 gives you a chemical yield of 66 per cent.
Calculating Chemical Yield
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