- Define and explain Yield and Atom Economy in your own words
- Use key terms such as relative formula mass (Mr) accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Yield and Atom Economy: measure how efficient a reaction is.
Measure how efficient a reaction is.
Key ideas
Real reactions are never perfect
Percentage yield compares what you actually make with the theoretical maximum: (actual ÷ theoretical) × 100. Yields fall short because of reversible reactions, side reactions and losses in filtering or transfer. Atom economy goes further, asking what fraction of the reactant atoms end up in the desired product — important for sustainable industry.
The mole connects mass and particles
One mole of any substance contains 6.02 × 10²³ particles and has a mass in grams equal to its relative formula mass — so 18 g of water is one mole. The relationship moles = mass ÷ Mr lets you convert between the two in seconds. This is the key that unlocks every reacting-mass calculation.
Key term — relative formula mass (Mr): The sum of the relative atomic masses in a formula, e.g. Mr of H₂O = 18.
Explain why a chemist might prefer a reaction with 100% atom economy even if its percentage yield is lower.
High atom economy means most reactant atoms end up in the desired product rather than waste, saving raw materials and reducing waste disposal — even if some product is lost in practice.
Answer: High atom economy means most reactant atoms end up in the desired product rather than waste, saving raw materials and reducing waste disposal — even if some product is lost in practice.
- Giving percentage yield above 100% without questioning it A yield over 100% means something is wrong: the product is probably wet or impure, or a mass was mismeasured. Check your results rather than reporting an impossible yield.
- Forgetting to double the oxygen in O₂ Mr of O₂ is 32, not 16 — the molecule has two atoms. The same trap applies to H₂, N₂ and the halogens: read the formula, not just the element.
Practice
(6.72 ÷ 8.40) × 100 = 80.0%.
Mr = 23.0 + 16.0 + 1.0 = 40.0. Moles = 10.0 ÷ 40.0 = 0.250 mol.
Moles of Mg = 0.494; moles of O₂ = 0.494 ÷ 2 = 0.247 mol; mass = 0.247 × 32.0 = 7.90 g. (Check: 12.0 + 7.90 = 19.9 g of product.).
0.20 ÷ 0.50 = 0.40 mol/dm³.
Quick check
Which of these best defines "relative formula mass (Mr)"?
5.84 g of NaCl is dissolved to make 1.00 dm³ of solution. What is its concentration? (Mr of NaCl = 58.4)
- Yield and Atom Economy: measure how efficient a reaction is.
- Real reactions are never perfect: Percentage yield compares what you actually make with the theoretical maximum: (actual ÷ theoretical) × 100.
- mole: The amount of substance containing 6.02 × 10²³ particles (Avogadro's constant).
- Watch out for: giving percentage yield above 100% without questioning it