Percentage Yield & Atom Economy [Chemistry only]

Percentage yield

• The yield is the amount of product actually obtained. It is usually less than the calculated maximum because:
  • The reaction is reversible and does not go to completion
  • Some product is lost when it is separated from the reaction mixture, for example on filter paper
  • Some reactants react in unexpected ways to make other products

$$\text{\% yield} = \frac{\text{mass of product actually made}}{\text{maximum theoretical mass of product}} \times 100$$

• Worked example: a reaction should make 5.0 g of product and 3.8 g is collected. $\frac{3.8}{5.0} \times 100 = 76\%$
• (HT) The theoretical mass comes from a reacting mass calculation: moles of reactant, ratio, mass of product. Then the actual mass is compared with it
• Worked example (HT): 10.0 g of calcium carbonate is heated and 4.2 g of calcium oxide is collected. $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$
  • Moles of $\text{CaCO}_3 = \frac{10.0}{100} = 0.100$ mol, so 0.100 mol of CaO, theoretical mass $0.100 \times 56 = 5.6$ g
  • Yield $= \frac{4.2}{5.6} \times 100 = 75\%$