Percent Yield & Percent Purity

A: Percent Yield
-A percent yield in chemical equations is used because chemical reactions generally                        
  do not produce the predicted amount of a substance predicted by chemists. 
  The percent yield is generally determined by the masses that are used in a chemical reaction as  
   well as by determining the mole ratio

>Ratio of amount of product obtained to amount of product expected by calculation, expressed as %
                                            ________________________________
                           %Yield  = |grams of actual product recovered            |   X   100%
                                            |grams of product expected from stoichi    |
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Example: If you burn 12 grams of carbon to make CO₂, then amount of carbon dioxide expected is one mol of CO₂ or 44 grams of CO₂. Sadly the amount you will get will probably be less than 44 grams and more like 34 grams of CO₂. The problem is a competing reaction that happens. Some carbon reacts to make CO.

2 C(s) + O₂(g) --- > 2 CO(g)

The carbon participating in this "side" reaction will not be able to make CO₂. The reaction will not yield 100% of the expected CO₂.
The amount of carbon dioxide produced, 34 grams of CO₂ is only 77% and not 100 % of the expected 44 grams.   


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B: Percent Purity
-Calculate how much reactant that actually is available to react        

                           %Purity  = Mass of Pure Substance    =x 100%

                                            Mass of Impure sample

  

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