Percent Purity

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 Many samples of chemicals are not pure. We can define percent purity as

mass of pure compound in the impure sample     x 100%

total mass of impure sample             

If an impure sample of a chemical of known percent purity is used in a chemical reaction, the percent purity has to be used in stoichiometric calculations. Conversely, the percent purity of an impure sample of a chemical of unknown percent purity can be determined by reaction with a pure compound as in an acid-base titration. Percent purity can also be determined, in theory, by measuring the amount of product obtained from a reaction. This latter approach, however, assumes a 100% yield of the product.

Examples

Consider the reaction of magnesium hydroxide with phosphoric acid.

3Mg(OH)2  +  2H3PO4 ---->   Mg3(PO4)2 + 6H2O

(a) Calculate the mass of Mg3(PO4)2 that will be formed (assuming a 100% yield) from the reaction of 15.0 g of 92.5% Mg(OH)2 with an excess of H3PO4

mass Mg(OH)2 = 15.0 x 0.925 = 13.875 g

mass Mg3(PO4)2 =

13.875 g Mg(OH)2 x  1 mol Mg(OH)2  x  1 mol Mg3(PO4)2  x  262.9 g Mg3(PO4)2

                         58.3 g Mg(OH)2       3 mol Mg(OH)2        1 mol Mg3(PO4)2

=  20.9 g Mg3(PO4)2

(b) Calculate the mass of 88.5% Mg(OH)2 needed to make 127 g of Mg3(PO4)2, assuming a 100% yield.

 mass Mg(OH)2 =

127 g Mg3(PO4)2 x  1 mol Mg3(PO4)2  x  3 mol Mg(OH)2  x  58.3 g Mg(OH)2

                        262.9 g Mg3(PO4)2   1 mol Mg3(PO4)2  1 mol Mg(OH)2

= 84.49 g Mg(OH)2.

mass 88.5% Mg(OH)2  =

84.49 g Mg(OH)2   x  100 g 88.5% Mg(OH)2  = 95.5 g

                                       88.5 g Mg(OH)2

(c) Calculate the percent purity of a sample of Mg(OH)2 if titration of 2.568 g of the sample required 38.45 mL of 0.6695 M H3PO4. 

mass Mg(OH)2 =

38.45 mL H3PO4 x  0.6695 mole H3PO4  x  3 moles Mg(OH)2

                                   1000 mL H3PO4            2 moles H3PO4

                                x     58.3 g Mg(OH)2

                                      1 mole Mg(OH)2

= 2.251 g Mg(OH)2

Percent Purity = 2.251   x  100% =  87.7%

                        2.568