How to Calculate Theoretical Yield
Calculate theoretical yield from a balanced equation, molar masses, and the limiting reactant, then compare it with percent yield.
Theoretical yield is the maximum product a reaction can make under ideal conditions. Convert the limiting reactant to moles, use the balanced-equation mole ratio, then convert moles of product to grams. In short: moles reactant → moles product → grams product.
After you find theoretical yield, use percent yield to compare the actual product with the ideal amount.
What theoretical yield means
Theoretical yield is a best-case calculation. It assumes the reaction goes to completion, no product is lost, and the measured reactant amounts are correct.
The limiting reactant sets the ceiling. A reaction cannot make more product than the first reactant to run out allows. If more than one reactant amount is given, identify the limiting reactant before calculating yield.
The formula behind the calculation
Theoretical yield is a chain of unit conversions. Start with grams of reactant, divide by its molar mass, multiply by the mole ratio, then multiply by the product's molar mass.
The mole ratio comes from the balanced equation. Do not use the subscripts inside a chemical formula as the reaction ratio.
Full worked example
Use calcium carbonate reacting with hydrochloric acid to form calcium chloride. The balanced equation shows a one-to-one ratio between calcium carbonate and calcium chloride.
Convert the starting material to moles.
Use the balanced-equation ratio, then convert to grams of product.
How to find the limiting reactant
When two reactant amounts are given, calculate how much product each one could make. The smaller product amount is the theoretical yield because that reactant runs out first.
This extra step matters. Using the non-limiting reactant overstates the possible product.
Theoretical yield versus percent yield
Theoretical yield is the ideal amount. Actual yield is what you collected. Percent yield compares the two.
A percent yield below one hundred is common because product can be lost during transfer, drying, filtration, or side reactions.
Common mistakes
Skipping the balanced equation breaks the mole ratio. Balance first, then read the coefficients.
Using grams directly in the ratio gives the wrong result. Reaction ratios compare moles, not grams.
Ignoring the limiting reactant overstates yield. If one reactant runs out, extra amounts of the other reactant cannot make more product.
Rounding molar masses too early can move the final answer. Keep a few extra digits until the final grams.
Common questions
Theoretical yield is the maximum amount of product a reaction can produce under ideal conditions, based on the balanced chemical equation and the limiting reactant.
Convert the limiting reactant to moles, use the balanced-equation mole ratio to find moles of product, then multiply by the product's molar mass.
Actual yield is usually lower because product can be lost, side reactions can occur, and reactions may not go fully to completion.
It can be reported in either, but chemistry lab problems usually ask for grams of product. Convert moles of product to grams with the product's molar mass.
Theoretical yield is a stoichiometry conversion with one controlling idea: the limiting reactant sets the maximum. Balance the equation, stay in moles for the ratio, and convert back to grams at the end.
