Percent Yield & Percent Purity

Percent Yield
Percent yield is the ratio between how much of the product was actually used, over what amount should be produced according to stoichiometry, all multiplied by 100.
This can be written as:

                      {grams of actual product obtained}                        *100%

                      {grams of product expected from stoichiometry}

This can be used to find the actual amount of chemical that goes through the reaction. The higher percent, the better.
Example problem
starting with the reaction
____ Mg + ____ HNO₃ à ____ Mg(NO₃)₂ + ____ H_{2
Balance it, and find the percent yield if you start with 32.1g of Mg, and 190.0g of Mg(NO3)2 is produced find the percent yield of} Mg(NO₃)_2.
Step 1: 1 Mg + 2 HNO₃ à 1 Mg(NO₃)₂ + 1 H₂
Step 2: 32.1 g Mg * 1 Mol Mg * 1 Mol Mg(NO3)2 * 148.3g Mg(NO3)2     =            195.9g Mg(NO3)2
                               24.3g Mg        1 Mol Mg          1 Mol Mg(NO3)2
Step 3: 190.0g of Mg(NO3)2 * 100% = 97.0%
            195.9g of Mg(NO3)2
Please note: always round to 1 decimal place.
Extra Help
For some extra practice, check out this website
http://www.uen.org/utahlink/tours/tourElement.cgi?element_id=42288&tour_id=17891&category_id=33176
Or, for more explanations check out this video
http://www.youtube.com/watch?v=Kk5kZO8ueWI

Percent Purity
Percent purity is used to calculate of much of the reactant within a substance is actually able to react.
Can be calculated using this formula:

                                                   {Mass of Pure Substance}          *   100

                                                   {Mass of Impure Sample}

Percent purity can also be used in reverse order, as long as two of of the three unknowns are given.
Example Problem:
112.0 g of solid was obtained, but analysis showed that only 105.6g of it was the actual product. Calculate the percent purity of the product.
105.6g product  *100 = 94.3%
112.0g product
Once again round to 1 decimal place.