(DA) Jan. 7, 2011: 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:

• Determine the percent by mass of Carbon in Octane (C8H18)

 8     =    30.8% Carbon             96     =    84% Hydrogen
26                                             114

• Determine the percent of mass of each element in Zinc Chloride ZnCl2   

65.4+71             Zn 65.4 = 47.9%
                              136.4      
                                                                            
                          Cl  71  = 52.1%
                              136.4             

*Keep to 3 decimal places!