Percentage Yield
YIELD is the amount of PRODUCT that a reaction forms.
When a reaction is carried out, the YIELD you ACTUALLY get is usually different to the amount you EXPECT to get.
- THEORETICAL YIELD is the maximum amount of product estimated to be produced. This can be found using mole calculations.
- ACTUAL YIELD is the amount of product that is actually produced. This can be found by carrying out the experiment in real life,
PERCENTAGE YIELD compares theoretical yield with actual yield and is given by the formula: 
In calculations to find percentage yield, the ACTUAL YIELD is usually given in the question, and the THEORETICAL yield needs to be found using MOLE CALCULATIONS.
Here's an example:
PERCENTAGE YIELD is always somewhere between 0 and 100%. 100% yield means that you got all the product you expected to get. 0% yield means that no reactants were converted into product.
Industrial processes should have as high a percentage yield as possible to reduce WASTE and reduce COSTS.
In real life, 100% YIELD IS NEVER ACHIEVED. Some product or reactant always gets lost along the way — and that goes for industrial processes as well as school lab experiments. How this happens depends on what sort of reaction it is and what apparatus is being used.
Why percentage yield is not 100%
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Not All Reactants React to Make a Product: In REVERSIBLE REACTIONS, the products can turn back into reactants, so the yield will never be 100%.
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Side Reactions: The reactants sometimes react differently to how you expect eg. react with gas in the air
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Loss of Product: When you filter a liquid to remove solid particles you always lose some material