All question types · Stoichiometry
Atom economy
A-Level (AQA) · unreviewed
Balanced equation with relative formula masses of every species; C is the desired product.
Worked example
For A + 2B → 2C + D (Mr: A 156.6 g mol⁻¹, B 79.3 g mol⁻¹, C 75.2 g mol⁻¹, D 164.8 g mol⁻¹), what is the atom economy of the process if C is the only useful product?
1. Multiply by a coefficient: 2 × Mr(C) → Mass of desired product
= 150.4 g mol⁻¹
2. Weighted sum of 1·Mr(A) + 2·Mr(B) → Total mass of reactants
= 315.2 g mol⁻¹
3. Divide Mass of desired product ÷ Total mass of reactants → Atom economy
= 0.4772
Answer: Atom economy = 0.477
Method
Atom economy = (mass of desired product ÷ total mass of reactants) × 100, using the coefficients: 2Mr(C) ÷ [Mr(A) + 2Mr(B)]. Because mass is conserved, the bottom line also equals 2Mr(C) + Mr(D).
Common mistakes
- Use the equation’s coefficients: 2 × Mr(C) over Mr(A) + 2 × Mr(B).
- Atom economy compares the desired product with all the reactants (equivalently, all the products), not with the by-product.
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