Explain how the law of conservation of mass is consistent with the concept of mole ratios in a balanced chemical equation, and outline how atom economy differs from percentage yield as a measure of the efficiency of a chemical reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The law of conservation of mass states that mass (or atoms) is neither created nor destroyed in a chemical reaction, so the total mass of reactants equals the total mass of products.
Mole ratios in a balanced equation are fixed proportions that show how many moles of each species are involved. Because the equation is balanced, the number of atoms of each element is the same on both sides, which directly reflects the conservation of mass.
Atom economy is calculated as (molar mass of desired product ÷ total molar mass of all products) × 100 %. It is a theoretical value that indicates how much of the reactants’ mass ends up in the useful product, independent of experimental conditions.
Percentage yield is (actual mass of product ÷ theoretical mass from stoichiometry) × 100 %. It measures the efficiency of a particular experiment, taking into account losses, whereas atom economy shows the inherent wastefulness of the reaction pathway itself.
Mole ratios in a balanced equation are fixed proportions that show how many moles of each species are involved. Because the equation is balanced, the number of atoms of each element is the same on both sides, which directly reflects the conservation of mass.
Atom economy is calculated as (molar mass of desired product ÷ total molar mass of all products) × 100 %. It is a theoretical value that indicates how much of the reactants’ mass ends up in the useful product, independent of experimental conditions.
Percentage yield is (actual mass of product ÷ theoretical mass from stoichiometry) × 100 %. It measures the efficiency of a particular experiment, taking into account losses, whereas atom economy shows the inherent wastefulness of the reaction pathway itself.
Examiner tips
- Use the exact definition of atom economy and percentage yield; include the ×100 % factor.
- Show the link between balanced equations and equal atom counts to satisfy the conservation law.
- Mention that atom economy is theoretical, yield is experimental.
- Keep the answer to 4 points – one sentence per point.
Common mistakes
- Confusing atom economy with yield or using the wrong formula.
- Failing to state that atom economy is independent of experimental conditions.
- Not recognising that the law of conservation is reflected in the balanced equation’s equal atom numbers.
Mark scheme (4 marks)
- The law of conservation of mass states that mass (or atoms) is neither created nor destroyed in a chemical reaction, so the total mass of reactants equals the total mass of products.
- Mole ratios in a balanced equation reflect the fixed proportions in which atoms are rearranged, ensuring that the number of atoms of each element is the same on both sides, which is consistent with conservation of mass.
- Atom economy is defined as the molar mass of the desired product divided by the total molar mass of all products (multiplied by 100%), and is a theoretical value intrinsic to the reaction pathway chosen.
- Percentage yield compares the actual experimental amount of product obtained to the theoretical maximum calculated from stoichiometry, so it reflects real experimental losses, whereas atom economy reflects how wasteful the reaction is in principle regardless of experimental conditions.
Key terms in this question
law of conservation of mass · mole ratio · atom economy · percentage yield
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
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- Decode the mark scheme abbreviations →
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