Source: Elements of Modern Chemistry, Charles Adolphe Wurtz, 1887.
The theoretical yield of a chemical reaction is the amount of product you expect to get after a chemical reaction takes place from the reagents you have available. To calculate the theoretical yield of a reaction, you must first know the reaction. Let’s look at the following reaction where heating potassium chlorate (KClO3) produces oxygen gas (O2).
2 KClO3 (s) → 3 O2 (g) + 2 KCl (s)
where KCl is potassium chloride. This reaction is fairly common in school laboratories since it is a relatively inexpensive method of obtaining oxygen gas.
The balanced reaction shows that 2 moles of KClO3 will produce 3 moles of O2 and 2 moles of KCl. To calculate the theoretical yield, you use these ratios as a conversion factor. Here is a typical example problem.
Calculate Theoretical Yield Example Problem
Question: How many moles of oxygen gas will be produced from heating 735.3 grams of KClO3?
Answer:
Step 1. We need to know the amount of KClO3 in moles to use the conversion, so the first step is to convert grams KClO3 to moles KClO3. To make this easier, know the molecular mass of KClO3 is 122.55 g/mol.
6 = x moles KClO3
Step 2: Use the chemical equation to relate moles KClO3 to moles O2.
From above, we see 2 moles of KClO3 will produce 3 moles of O2 gas.
x moles O2 = 3 x 3 moles O2
x moles O2 = 9 moles O2
6 moles of KClO3 (735.3 grams of KClO3) will produce 9 moles of O2 gas.