Worked example 1
A reaction has a theoretical yield of 12.5 g of aspirin, and 9.85 g is recovered. What is the percent yield?
Try it first: Estimate first: is the answer nearer 60 %, 80 % or 95 %?
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Percent yieldCourse level: High School / HonorsCourse level: AP Chemistry
Percent Yield
What you'll be able to do: Compare an actual laboratory yield with the theoretical yield and interpret what the percentage says about the experiment.
Best after: Limiting Reactant and Excess
Introduction
Calculations predict the maximum. Flasks deliver less. Percent yield is the honest comparison between the two, and learning to read it is how a calculation becomes an evaluation of an experiment rather than just an answer.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Distinguish theoretical, actual and percent yield.
- Calculate percent yield from experimental data.
- Work backwards from a percent yield to a predicted actual mass.
- Explain common physical reasons a yield falls below or appears above 100 %.
Three yields, three meanings
Theoretical yield is calculated from the limiting reactant and represents perfection. Actual yield is measured on a balance after the product is isolated and dried. Percent yield is the ratio of the two.
Only the actual yield is data; the other two are, respectively, a prediction and a judgement about the prediction.
The theoretical yield always comes from the limiting reactant, never from the excess reactant.
The calculation
Divide the actual yield by the theoretical yield and multiply by 100. Both quantities must describe the same product and use the same unit, so if one is in grams and the other in moles, convert first.
Because it is a ratio of two masses of the same substance, the units cancel and percent yield is dimensionless.
percent yield = (actual yield / theoretical yield) × 100 %
Why yields fall short
Product is lost when transferring between containers, some stays dissolved in the filtrate, side reactions consume reactants, the reaction may not reach completion, and purification always removes some product along with impurities. None of these is a mistake in the calculation, they are physical facts about doing chemistry.
A 70% isolated yield does not necessarily mean only 70% of the reactant reacted. Complete chemical conversion followed by product loss during filtration can give the same result as incomplete conversion with perfect recovery. Percent yield measures the collected desired product relative to its theoretical maximum, combining reaction selectivity, completion and recovery. It cannot identify the cause of a low value by itself.
A good explanation of a low yield names a physical loss, not simply human error.
Yields above 100 %
A percent yield above 100 % is not possible for the pure product, so it is a signal. The usual causes are a product that is still wet with solvent, an impure product containing unreacted starting material, a mis-set limiting reactant, or an unbalanced equation.
Treat it as a diagnostic prompt to re-check both the calculation and the drying step.
Above 100 % means something is wrong or something extra was weighed.
Working backwards
If a reaction is known to run at, say, 78 % yield, you can predict the product mass: actual = 0.78 x theoretical. Chemists also invert this to decide how much starting material to buy for a target amount of product, which is the everyday industrial use of this idea.
Planning often continues back to a reactant. For CaCO₃ → CaO + CO₂, suppose 10.0 g CaO is required at 80.0% isolated yield with all other assumptions unchanged. First require 10.0/0.800 = 12.5 g theoretical CaO. Convert to moles, 12.5/56.08 = 0.2229 mol CaO; the 1:1 ratio requires 0.2229 mol CaCO₃, or 22.3 g using 100.09 g/mol. Multiplying 10.0 g by 0.800 would plan for less product than the target, so dividing is the physically sensible direction. The yield estimate must apply to the same reaction and recovery procedure.
actual yield = (percent yield / 100) x theoretical yield
Trace an error to the numerator or denominator
A percent yield is actual divided by theoretical, so an error's direction depends on which quantity it changes. A wet or contaminated product raises the measured actual mass and inflates the reported percent. Losing solid during transfer lowers actual mass and the percent. Overestimating the limiting reactant raises the calculated theoretical yield and lowers the percent even if the collected mass is unchanged. Name the affected measurement first, then follow the ratio. A value under 100% alone does not tell you which loss mechanism occurred.
| Observation or error | Quantity directly affected | Reported yield direction |
|---|---|---|
| Product remains wet | Actual mass too large | Too high |
| Some product passes through the filter | Actual mass too small | Too low |
| Starting reactant amount overestimated | Theoretical yield too large | Too low |
| Wrong stoichiometric coefficient used | Theoretical yield incorrect | Determine from the specific ratio |
A measured value below 100% can still be inflated by moisture; an error need not push the final number above 100% to matter. Distinguish an 8 percentage-point change from an 8% relative change when comparing two reported yields.
Key ideas
Definition
Theoretical yield
The maximum product mass predicted from the limiting reactant, assuming complete reaction and no losses.
Definition
Actual yield
The mass of purified product actually recovered and weighed.
Definition
Percent yield
Actual divided by theoretical, times 100. Dimensionless.
Rule
Same substance, same units
Both yields must refer to the same product and be expressed in the same unit before dividing.
Assumption
The product is pure and dry
Percent yield only means what it claims if the weighed sample is the isolated product, not product plus solvent.
Equation
Percent yield
percent yield = (actual / theoretical) × 100 %
- actual = measured mass (or moles) of purified product
- theoretical = maximum predicted from the limiting reactant, same units
- use when = an experimental product mass is reported alongside a calculation
- limits = values above 100 % indicate impurity or a calculation error, not a better reaction
Equation
Predicted actual yield
actual = (percent yield / 100) x theoretical
- percent yield = an efficiency known from previous runs
- use when = planning how much product a given amount of reactant will really deliver
- limits = assumes the new run behaves like the previous ones
Worked examples
Worked example 2
Burning 25.0 g of C₃H₈ in excess O₂ gives 68.2 g of CO₂. What is the percent yield? C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O
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Worked example 3
A synthesis reliably runs at 65.0 % yield. If the theoretical yield is 40.0 g, how much product should you expect, and how much would you need theoretically to obtain 40.0 g of product?
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Worked example 4
The theoretical yield is 5.00 g. A collected product weighs 4.60 g while wet and 4.20 g after drying. What are the two apparent percent yields?
Try it first: Use the same theoretical mass with each measured actual mass.
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Common mistakes
| Mistake | Why it's wrong | Check instead |
|---|---|---|
| Dividing theoretical by actual. | That gives a value above 100 % for any normal experiment and reverses the meaning of the number. | Actual on top, always. The measured value is the numerator. |
| Calculating the theoretical yield from the excess reactant. | The excess reactant is still present when the reaction stops, so the prediction is too large and the percent yield is falsely low. | Identify the limiting reactant before computing any theoretical yield. |
| Dividing a mass by a mole value. | The units do not cancel, so the result is not a percentage. | Convert both to the same unit for the same substance first. |
| Reporting a yield above 100 % as a good result. | You cannot recover more pure product than the limiting reactant can supply, so it points to a wet or impure sample or a calculation error. | Re-check the limiting reactant, the balanced equation, and whether the product was dried. |
| Explaining a low yield only as human error. | Most losses are inherent to the procedure, such as transfer losses and solubility in the filtrate. | Name a specific physical loss step when asked to explain a low yield. |
Scroll horizontally if needed to read all columns.
After the lesson · Question bank
Independent practice
Work through new problems on your own. The lesson's quick checks and worked examples stay above if you need to review a step.
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What you should now know
Percent yield is the actual yield divided by the theoretical yield, times 100. The theoretical yield always comes from the limiting reactant. Both masses must be for the same substance and in the same units, and a value above 100 % signals an error or an impure product rather than an unusually good reaction.
- Percent yield = actual / theoretical x 100, with both for the same product in the same units.
- Theoretical yield always comes from the limiting reactant.
- Yields below 100 % are normal and usually reflect transfer, solubility and purification losses.
- A yield above 100 % is a diagnostic signal of impurity, moisture, or a calculation error.
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Sources and further reading
- Chemistry 2e, Section 4.4: Reaction YieldsPaul Flowers, Klaus Theopold, Richard Langley, William R. Robinson · OpenStax, Rice University · Chapter 4.4View source
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Source rights and review status - General Chemistry bookshelf (LibreTexts)LibreTexts, University of California, DavisView source
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