If all the reactants become products, the percentage yield is 100%. If half the reactants become products and the rest become by-products or don't react at all, the percentage yield is 50%. If none of the reactants end up as products, the percentage yield is 0%.
chemical reactions very often form by-products as well as the intended product
in most reactions, not all of the reactants actually react
What is left at the end of a reaction is thus typically a mixture of product and impurities (impurities being by-products and unreacted starting materials).
Achieving high percentage yields is particularly important in industries that rely on organic synthesis, such as the pharmaceutical industry. It may require tens of steps to synthesise a drug molecule — if there are ten steps, each of which has a percentage yield of 90%, the overall yield is only 35%:
There is a simple formula you can use to calculate the percentage yield of a reaction A → B:
Percentage yield effectively states the percentage of moles of reactant that become product — although this definition only applies if one mole of reactant ideally forms one mole of product, i.e. 1:1 stoichiometry.
In practice, the number of moles of A and B involved have to be calculated using the masses, m and molar masses, M, of A and B.
stands for the mass of substance A in g
stands for the molar mass of substance A in g mol−1
stands for the mass of substance B in g
stands for the molar mass of substance B in g mol−1
In words, this is:
This can also be written as:
Another way of writing the same this is given below, but this form is a little untidy: