Percentage Yield and Atom Economy

Chemists want to make useful products efficiently. Two measures help judge this: percentage yield (how much you actually make) and atom economy (how many atoms end up in the product you want). Both are used in labs and industry.

Percentage Yield

Idea: Not all reactions produce the full, calculated amount. Percentage yield compares what you got to what you could have got in perfect conditions.

$$\text{percentage yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100\%$$

• Actual yield: the mass you really collect.
• Theoretical yield: the maximum mass predicted by the balanced equation if nothing is lost.

Why yields are below 100%: some reactants do not react, side reactions happen, product is lost during filtering or transferring, or reactants are impure.

Atom Economy

Idea: Atom economy measures how many of the atoms from the reactants end up in the desired product (not in by‑products). Think of the products as a pie; atom economy is the slice that is the product you want.

$$\text{atom economy} = \frac{M_r\ \text{of desired product}}{\sum M_r\ \text{of all products}} \times 100\%$$

• High atom economy means less waste and greener chemistry.
• Reactions with a single product often have 100% atom economy.

Why these measures matter

• Laboratory planning: percentage yield helps judge how successful a practical was.
• Industry and the environment: high atom economy reduces waste, saves resources, and lowers costs.

Common misconceptions

• 100% yield does not always mean no waste; there may still be by‑products.
• Atom economy uses the products’ $M_r$, not the reactants’.
• Yield depends on experiment and technique; atom economy depends only on the balanced equation.