Chapter 8 · Quantities in Chemical Reactions
8.5More Pancakes: Limiting Reactant, Theoretical Yield, and Percent Yield
12 min · two checks
Predict
If the balanced recipe can make 12.0 g of product and you isolate 9.0 g, what is the percent yield?
The idea
- Define limiting reactant, theoretical yield, actual yield, and percent yield.
- Compute percent yield from those masses.
The limiting reactant is consumed first and determines how much product can form. Any other reactant is in excess. The theoretical yield is the product mass calculated from the limiting reactant, assuming the reaction goes perfectly to completion. The actual yield is what you isolate in the lab, almost always less, because of incomplete reaction, side reactions, and material left in glassware.
Percent yield = (actual yield / theoretical yield) × 100%. Both yields must be in the same unit. A percent yield above 100% means a calculation error or a wet, impure product, not a victory over conservation of mass. You cannot compute percent yield from the excess reactant. Using the wrong reactant silently inflates the theoretical yield and crushes the percent.
Keep these
- Limiting reactant sets the theoretical yield.
- Percent yield = actual / theoretical × 100%.
- Yields above 100% are a signal to recheck, not a super-reaction.