Monday, April 4, 2011

Percent Purity

Reactants that are used in experiments/reactions may not be 100% pure, so we use percent purity calculations to calculate how much of the pure substance is in a reactant.

% Purity = mass of pure substance
                -------------------------- X 100
                 mass of impure sample

It is basically the ratio of mass of pure substance to the mass of the impure sample.

Ex/ If a 2.50g sample of iron ore contains 0.90g of Iron metal, what is the % purity?

0.90g iron
                                      --------------- x 100% = 36.0% Purity 
2.50g iron ore

Ex/ 2Na + 2H20 -> 2NaOH + H2

If a 7.5g sample of impure Na that is 75% pure is reacted, how many grams of H2 is produced?

75% = pure Na
          ----------            X 100
           7.5g impure Na

75 x 7.5g
----------- = grams of pure Na = 5.6g pure Na

      5.6g Na x 1 mole Na     1 mole H2        2.0g H2
                          ------------- x   ------------- x ------------- = 0.24 g H2 produced
                     23.0g Na       2 mole Na       1 mole h2

By the way, if your percent purity ends up being over 100%, you either did something incorrectly, or you have stumbled upon a trick question. Percent yield cannot be over 100% either, by the way.

Don't fry your brain. Enjoy this cute little illustration.

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