The amount of a product obtained is known as the yield. When compared with…

The idea

Yield is the actual quantity of a named product formed in a reaction, usually measured in grams. The theoretical maximum is the maximum mass of product that could possibly be created from the given reactants. Percentage yield compares the two: percentage yield = (actual yield ÷ theoretical yield) × 100. For example, if a reaction should theoretically make 8.0 g of copper sulfate but only 6.0 g is obtained, the percentage yield is (6.0 ÷ 8.0) × 100 = 75%. Losses arise from incomplete reaction, side reactions, transferring and filtering, evaporation and purification. Percentage yield is always less than 100% in practice, though a calculated value above 100% signals impure or wet product.

What earns the marks

• State that yield is the actual amount of product obtained, and identify the product and unit in a given context.
• Calculate percentage yield when given the actual yield and the maximum theoretical yield.
• Apply percentage yield = (actual yield ÷ theoretical yield) × 100, rearranging to find an unknown actual or theoretical yield.
• Interpret a percentage yield value, explaining that values below 100% arise from incomplete reaction, side reactions or product loss during separation and purification.
• Recognise that a value above 100% indicates impurities or residual solvent rather than a genuine extra amount of product.

Common mistakes and how to put them right

• Dividing theoretical yield by actual yield instead of actual by theoretical.

  Put it right by writing the fraction as actual yield over theoretical yield before multiplying by 100.

• Forgetting to multiply by 100 and reporting a decimal such as 0.75.

  Put it right by checking that a percentage yield is expressed on a 0 to 100 scale.

• Confusing the mass of the starting reactant with the theoretical yield of the product.

  Put it right: Carefully reading the question to identify the stated theoretical maximum mass of the product.

Exam tips

• Underline the actual yield and the theoretical yield in the question before substituting, so the fraction is set up the right way round.
• Show all working when substituting values into the percentage yield equation, as method marks are often available even if the final arithmetic slips.
• Check whether the question asks for a mass, a percentage or a reason for a low yield, and answer in that form with units where appropriate.