Calculating Yield
This compares the expected (theoretical) mass of product with the actual mass of product, giving the percentage figure for the reaction. The expected mass is worked out from the balanced equation:

An example
A method used in the past to manufacture phenol from benzene used sulphuric acid and sodium hydroxide in several steps.

The equation indicates that 1 mole of benzene (78g) should yield 1 mole of phenol (88g). Other reactions take place, however, and the actual mass of phenol will be in the region of 77g, giving a yield of about 88%.

The Problem of Yield

A high yield is desirable, but is not the whole story. The reaction once used to manufacture phenol from benzene generates 1 mole of sodium sulphate (IV) (sodium sulphate) (126g), for every mole of phenol (88g) produced. This may be acceptable if there is sufficient demand for the sodium sulphate (IV), but if not, it presents a serious waste management problem, and adds significantly to costs.
It is much better to reduce the amount of waste generated in the first place, rather than treat it at a later date, and this is an important aim of green chemistry. A yield calculation does not indicate how efficiently the reactants have been used in generating the desired product, and so does not indicate how effectively waste is being reduced.

Atom Economy

The concept of Atom Economy was developed by Barry Trost of Stanford University (US), for which he received the Presidential Green Chemistry Challenge Award in 1998. It is a method of expressing how efficiently a particular reaction makes use of the reactant atoms.

This approach does not take yield into account, and does not allow for the fact that many real-world processes use deliberate excess of reactants. It does, however, help in comparing different pathways to a desired product.

Examples

Benzene can be oxidized to make maleic anhydride, an important intermediate chemical.

The Atom Economy of this reaction is 43%, calculated using the relative formula masses. This means that 43% of the mass of the reactants ends up in the desired product.

Phenol has been produced by a variety of methods, and each alternative has shown an improved atom economy.

Ibuprofen became obtainable without prescription in the 1980s. At that stage the method used for production used six steps with an overall atom economy of just 40.1%. In the 1990s the Hoechst Celanese Corporation developed a new three-stage process with an atom economy of 77.4%, now a classic example of improving the route to a commercial product.