Percentage yield and atom economy
Chemistry · GCSE · The School
Percentage yield compares what you got to what was possible
Percentage yield = (actual mass obtained ÷ theoretical maximum mass) × 100. The theoretical maximum comes from the balanced equation, assuming every particle reacts perfectly with none lost. The actual mass is whatever a real experiment produces — always less, because real reactions are never perfect.
A worked calculation
A reaction has a theoretical maximum yield of 12 g. The student actually obtains 9 g. Percentage yield = (9 ÷ 12) × 100 = 75%. The missing 25% was not destroyed — it is somewhere in the equipment or the reaction mixture, not simply gone.
Why yield is never 100% in practice
Some product is left behind on filter paper or stuck to the sides of a beaker during transfer. Some reactions do not go to completion, or run alongside unwanted side reactions that use up the same starting materials. Some product may be lost during purification, such as when a solid is washed to remove impurities. None of these losses mean the chemistry was done wrong — they are normal, and a chemist plans for them.
Atom economy: a different question about the same reaction
Atom economy = (relative formula mass of the desired product ÷ sum of relative formula mass of all reactants) × 100. Unlike percentage yield, atom economy does not depend on how the experiment actually went — it is a property of the REACTION EQUATION itself, calculated before anything is even mixed.
A worked atom economy calculation
A reaction: A + B → C + D, where C is the desired product. If A has Mr 40, B has Mr 20, and C has Mr 44 (D, a by-product, has Mr 16), atom economy = (44 ÷ (40 + 20)) × 100 = 73.3%. A reaction making one product with no by-product at all has an atom economy of 100%.
Why atom economy matters for sustainability
A reaction can have a perfect 100% percentage yield and still waste most of its atoms if it also produces a large mass of unwanted by-product — that reaction has a low atom economy. High atom economy matters because it means less waste, better use of raw materials, and usually a cheaper, greener process — which is why industrial chemists often choose a lower-yielding route with a higher atom economy over the reverse.