Percentage yield · Year 12

In Year 12 you get fluent with: percentage yield, why yields are below 100%, and percentage yield from reacting masses.

A reaction rarely gives you all the product the equation says it could. The percentage yield compares the mass you actually collect with that maximum, the theoretical yield. Yields fall below 100% because reactions may not finish, product is lost when it is separated, and other reactions can happen.

Key facts

  • Percentage yield = mass of product made ÷ maximum (theoretical) mass × 100.
  • The theoretical yield is the maximum mass of product, worked out from the balanced equation and the mass of reactant used.
  • Yields are below 100% because the reaction may not finish, some product is lost when it is separated, and other reactions can happen.

Worked examples

Example 1

Heating some calcium carbonate could make 28 g of calcium oxide, but only 22.4 g is collected. Find the percentage yield.

Answer: 80%

Example 2

A reaction should make 12.5 g of copper sulfate crystals, but only 10.0 g is collected. Find the percentage yield.

Answer: 80%

Example 3

A reaction could make 27.5 g of product, but only 17.6 g is collected. What is the percentage yield?

Answer: 64%

Example 4

Give three reasons why the percentage yield of a reaction is less than 100%.

Answer: the reaction may be reversible and not go to completion; some product is lost when it is separated or moved from one container to another; other reactions may happen and make different products

Example 5

Nitrogen reacts with hydrogen to make ammonia: N2 + 3H2 ⇌ 2NH3. 14 g of nitrogen could make 17 g of ammonia, but only 2.55 g forms. Find the percentage yield, and give one reason it is so low.

Answer: 15%; the reaction is reversible, so it does not go to completion

Example 6

C + O2 → CO2. 6 g of chem C reacts and 15.4 g of chem CO2 is collected. What is the percentage yield, to 1 decimal place? (Aᵣ: C = 12, O = 16)

Answer: 70%

Example 7

Mg + 2HCl → MgCl2 + H2. 12 g of chem Mg reacts and 36.1 g of chem MgCl2 is collected. What is the percentage yield, to 1 decimal place? (Aᵣ: Mg = 24, H = 1, Cl = 35.5)

Answer: 76%

Other years