Percent Composition

The percentage by mass of an element in a compound is always the same.


To find the percent by mass determine the mass of each element present in one mole.


   Example: Hydrogen in Water (H20)


                  H=2.0g O=16.0g  18.0g/mol        2.0 / 18.0 = 0.111
                                                                                      = 11.1%


                  Sodium in Salt (NaCl)


                  Na=23.0g Cl=35.5g 58.5g/mol    23.0 / 58.5 = 0.393
                                                                                         = 39.3%