Atom economy calculator from the equation

Making lime from limestone: CaCO₃ → CaO + CO₂.

• $M_r$(CaCO₃) $= 40 + 12 + 3 \times 16 = 100$.
• $M_r$(CaO) $= 40 + 16 = 56$.
• Atom economy $= 56 \div 100 \times 100 = 56\%$. The other 44% leaves as carbon dioxide.

Making hydrogen from methane: CH₄ + H₂O → CO + 3H₂.

• Reactants: $16 + 18 = 34$. Desired product: $3 \times 2 = 6$.
• Atom economy $= 6 \div 34 \times 100 = 17.6\%$.

Include every coefficient: in the hydrogen example, the 3 in front of H₂ triples its mass. If the equation isn't balanced, the answer will be wrong, and if the desired product seems heavier than all the reactants together, that's a sign the equation isn't balanced. Our chemical equation balancer can check it first.

A high atom economy means less waste, which makes a process more sustainable and often cheaper. Reactions with only one product (addition reactions such as making ethanol from ethene and steam) have an atom economy of 100%. Industry also considers the yield, the rate, the cost of energy and whether by-products can be sold or used.

Atom economy comes from the equation alone: it's the best case. Percentage yield comes from what you actually collect. A process can score well on one and badly on the other, so exam questions often ask you to compare both. Work out yield with our percentage yield calculator.

• Leaving out a coefficient.
• Using the products' total on the bottom instead of the reactants' (in a balanced equation they're equal, but use the reactants as the specification does).
• Rounding $M_r$ values too early.

Practise with our atom economy questions.

• Write out each $M_r$ as a line of working before adding them.
• Read the question carefully for which product is wanted: making CO₂ from limestone would give a different atom economy.
• When comparing two methods, mention both atom economy and yield, and whether the by-product can be sold or used.