In chemistry, yield, also referred to as a reaction yield, is the amount of product obtained in a chemical reaction. The absolute yield can be given as the weight in grams or in moles (molar yield). The fractional yield, relative yield, or percentage yield, which serve to measure the effectiveness of a synthetic procedure, are calculated by dividing the amount of the obtained product by the theoretical yield (the unit of measure for both must be the same):
One or more reactants in a chemical reaction are often used in excess. The theoretical yield is something used in chemistry. Stochiometry is a branch of chemistry that deals with relative quantities of reactants and products in a chemical reaction. The theoretical yield is therefore calculated based on the molar amount of the limiting reactant, taking into account the stoichiometry of the reaction. For the calculation it is usually assumed that there is only one reaction involved.
The ideal or theoretical yield of a chemical reaction would be 100%, a value that is impossible to achieve due to limitations in measurement accuracy. According to Vogel's Textbook of Practical Organic Chemistry, yields around 100% are called quantitative, yields above 90% are called excellent, yields above 80% are very good, yields above 70% are good, yields above 50% are fair, and yields below 40% are called poor. It should however be noted that these names are arbitrary and not universally accepted, and for many reactions these expectation may be unrealistically high. Yields may appear to be above 100% when products are impure, as the measured weight of the product will include the weight of any impurities. Purification steps always lower the yield and the reported yields usually refer to the yield of the final purified product.
See also 
- Vogel, A.I., Tatchell, AR., Furnis, B.S., Hannaford, A.J. and P.W.G. Smith. Vogel's Textbook of Practical OrganicHall, 19936-3.
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