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How much product you actually get compared with the maximum possible, and how much of the starting material ends up in the product you want.
When you make something in chemistry, you never quite get as much as you hoped for. Some is always lost. Percentage yield tells you how much you got compared with the most you could have got.
Atom economy asks a different question. It asks how much of the starting material ends up in the product you want. Companies care about both, because waste costs money and harms the environment.
By the end you will be able to calculate a percentage yield and explain why it is below 100%. You will work out a theoretical yield from an equation. You will also calculate atom economy and say why it matters.
Course note: percentage yield and atom economy are Separate Chemistry content. Combined Science courses meet the ideas behind them: reversible reactions, waste and sustainability. Use this lesson if you have been taught the calculations.
The yield is the mass of product you collect. The theoretical yield is the maximum mass that could form. You work it out from the balanced equation, assuming nothing is lost. The actual yield is what you really get.
Percentage yield = actual yield ÷ theoretical yield × 100. It works like a test score. If the theoretical yield is 8.0 g and you collect 6.0 g, then 6.0 ÷ 8.0 × 100 = 75%.
A percentage yield can never be more than 100%. If you get more, check your working. The product may be wet or impure.
There are several reasons. The reaction may be reversible, so it does not go all the way to products. Some reactants may react in a different way to make other products. These are called side reactions.
Some product is also lost on the way. It may be left on the filter paper or stuck to the equipment. Think of baking, where some cake mixture always stays in the bowl.
The reactants might also be impure. In an answer, name the reason that fits the question. Do not just say errors.
To find a theoretical yield, follow three steps. Change the mass of the limiting reactant into moles. The limiting reactant is the one that runs out first. Use the ratio in the equation. Then change the moles of product into a mass.
Moles = mass ÷ Mr, where Mr is the relative formula mass. So mass = moles × Mr.
Heat 5.0 g of calcium carbonate: CaCO₃ → CaO + CO₂. Moles of CaCO₃ = 5.0 ÷ 100 = 0.050 mol. The ratio is 1 : 1, so 0.050 mol of CaO forms. Mass of CaO = 0.050 × 56 = 2.8 g.
Atom economy tells you how much of the mass of the reactants ends up in the useful product. Think of peeling an orange. The part you eat is the useful product. The peel is waste.
Atom economy = Mr of desired product ÷ sum of Mr of all reactants × 100. Use the balanced equation. Multiply each Mr by the number in front of it.
A reaction can have a high yield and a low atom economy. This happens when it also makes lots of waste products.
Iron is made by Fe₂O₃ + 3CO → 2Fe + 3CO₂. The desired product is 2 × 56 = 112. The reactants add up to 160 + 3 × 28 = 244.
Atom economy = 112 ÷ 244 × 100 = 45.9%. More than half the mass is wasted as carbon dioxide.
Some reactions have a 100% atom economy. This happens when there is only one product, as in an addition reaction. Ethene and steam make ethanol: C₂H₄ + H₂O → C₂H₅OH. Every atom ends up in the ethanol.
A good process has a high yield, a high atom economy, a fast rate and low costs. These can pull in different directions. A reaction with a high atom economy may be slow. A fast one may have a low atom economy.
A high atom economy means less waste. It uses fewer raw materials and costs less to dispose of waste. This makes a process more sustainable. A low atom economy wastes resources, which are often limited.
Companies also think about energy costs, safety and whether the by-products have a use. When you compare two processes, mention yield, atom economy, cost and rate.
Percentage yield = actual yield ÷ theoretical yield × 100. It cannot be more than 100%.
Yields are lower because of reversible reactions, side reactions and lost product.
To find a theoretical yield, change mass to moles, use the ratio, then change moles to mass.
Atom economy = Mr of desired product ÷ sum of Mr of reactants × 100. A high atom economy means less waste.
A student expects to collect 10 g of a product but collects 8.0 g. Calculate the percentage yield.
Answer: 80%
Calculate the atom economy for making hydrogen in CH₄ + H₂O → CO + 3H₂. Mr of CH₄ = 16, H₂O = 18 and H₂ = 2.
Answer: 18%
10.0 g of calcium carbonate is heated: CaCO₃ → CaO + CO₂. 4.2 g of calcium oxide is collected. Calculate the percentage yield. Mr of CaCO₃ = 100 and Mr of CaO = 56.
Answer: 75%
Percentage yield and atom economy practice questions for GCSE Combined Science Chemistry. Try each one, then open the answer. Easy questions come first.
The theoretical yield is 20 g and the actual yield is 15 g. Calculate the percentage yield.
Answer: 75%
Give one reason why the actual yield is less than the theoretical yield.
Answer: some product was lost
What is the atom economy of a reaction that makes only one product from its reactants?
Answer: 100%
What is the name for the maximum mass of product that can be made?
Answer: theoretical yield
What is the name for the mass of product that you actually collect?
Answer: actual yield
The theoretical yield is 10 g and the actual yield is 6 g. Calculate the percentage yield.
Answer: 60%
A student collects 3.6 g of a product. The theoretical yield is 4.5 g. Calculate the percentage yield.
Answer: 80%
Calculate the atom economy for making CaO from CaCO₃ → CaO + CO₂. Mr of CaCO₃ = 100 and Mr of CaO = 56.
Answer: 56%
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
A student heats 5.0 g of copper carbonate: CuCO₃ → CuO + CO₂. The theoretical yield of copper oxide is 3.2 g. (a) The student collects 2.4 g of copper oxide. Calculate the percentage yield. [2 marks] (b) Give two reasons why the percentage yield is less than 100%. [2 marks]
Answer: (a) 75% (b) Some product was lost when it was transferred, and some of the carbonate may not have decomposed.
Hydrogen can be made by the reaction CH₄ + H₂O → CO + 3H₂. Mr of CH₄ = 16, H₂O = 18 and H₂ = 2. (a) Calculate the atom economy of the reaction for hydrogen. [3 marks] (b) Hydrogen can also be made by electrolysing water: 2H₂O → 2H₂ + O₂. Explain why the atom economy for hydrogen from this reaction is lower than 100%. [2 marks]
Answer: (a) 17.6% (b) Oxygen is also made as a second product, so not all the mass of the water ends up as hydrogen.
Putting the theoretical yield on the top of the percentage yield fraction
The actual yield goes on the top. The result cannot be over 100%.
Confusing yield with atom economy
Yield is about how much you collect. Atom economy is about how much of the mass of the reactants is in the useful product.
Forgetting the big numbers when working out atom economy
Multiply each Mr by the number in front of it in the balanced equation.
Saying a yield below 100% breaks the law of conservation of mass
Mass is conserved. Some product was lost, or the reaction did not go to completion.
Saying a percentage yield of 110% shows a very efficient reaction
A yield cannot be over 100%. The product is probably wet or impure, so check the working.
Percentage yield and atom economy is on both Foundation and Higher papers for GCSE Combined Science Chemistry, aimed at around grade 6.
Putting the theoretical yield on the top of the percentage yield fraction. The actual yield goes on the top. The result cannot be over 100%.
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.
See how the papers are set: AQA Combined Science papers and Edexcel Combined Science papers.
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