Percentage yield
The theoretical yieldThe maximum possible mass of a product that can be made in a chemical reaction. is the maximum possible massThe amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). of a productA substance formed in a chemical reaction. that can be made in a chemical reaction.
It can be calculated from:
- the balanced chemical equationA chemical equation written using the symbols and formulae of the reactants and products, so that the number of units of each element present is the same on both sides of the arrow.,
- the mass and relative formula massThe sum of the relative atomic masses of the atoms in a chemical formula. of the limiting reactantThe reacting substance that is completely used up in a chemical reaction and which determines how much product is made., and
- the relative formula mass of the product
Even though no atoms are gained or lost in a chemical reaction, it is not always possible to obtain the calculated amount of a given product. Reasons why the mass of product made is less than the maximum theoretical mass include:
- the reaction not going to completion, because it is reversible
- some of the product may be lost when it is separated from the reaction mixture by filteringThe process of passing a mixture through a device - soluble substances pass through the filter as a 'filtrate' but insoluble substances or unwanted material will stay in the filter as a 'residue'., for example
- some of the reactants may react in ways different to the expected reaction
Calculating percentage yield
The percentage yieldA measure of how much product is actually made, calculated from: 100 × (actual yield) ÷ (theoretical yield). is calculated using this equation:
The percentage yield can vary from 100% (no product has been lost) to 0% (no product has been made).
Worked example
Copper oxide reacts with sulfuric acid to make copper sulfate and water. In an experiment, 1.6 g of dry copper sulfate crystals are made. If the theoretical yield is 2.0 g, calculate the percentage yield of copper sulfate.
Actual yield = 1.6 g
Percentage yield = \(\frac{\textup{1.6}}{\textup{2.0}}\) × 100
Percentage yield = 80%
Question
In an experiment, the theoretical yield is 3.2 g but the actual yield is only 2.4 g. Calculate the percentage yield.
Percentage yield = \(\frac{\textup{2.4}}{\textup{3.2}}\) × 100
Percentage yield = 75%