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ChemistryGCSE, IGCSE & A Level

What is percentage yield?

Percentage yield compares the mass of product actually obtained with the theoretical maximum predicted by the balanced equation.

Percentage yield explained

Yields fall short because reactions can be reversible, side reactions compete, and product is lost during filtration, transfer and purification.
Atom economy is a different measure: it asks what fraction of the reactant mass ends up in the desired product, and it can be calculated before any experiment.

Key formula

% yield=actual masstheoretical mass×100\% \text{ yield} = \dfrac{\text{actual mass}}{\text{theoretical mass}} \times 100

Worked example

Obtaining 6.4 g6.4\,\text{g} when theory predicts 8.0 g8.0\,\text{g} is an 80%80\% yield.

Examiner tip

A yield above 100% means the product is impure or still wet — say so explicitly if a question presents that data.

Percentage yield: common questions

What is the difference between yield and atom economy?
Yield measures how much product you actually recovered; atom economy measures how much of the reactant mass is designed to become that product.
Why is a high atom economy important industrially?
It means less waste, lower raw-material cost and fewer by-products to separate and dispose of.

Related terms

Limiting reagent

The limiting reagent is the reactant that is fully used up first and therefore determines the maximum yield of product.

Mole

A mole is the amount of substance containing 6.02 × 10²³ particles — the chemist's counting unit that links mass to number of particles.

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