Percent Composition

       To find the  Percent Composition you first have to find the Formula weight: the sum of the atomic mass for the atoms in a chemical formula. Then you divide part over whole times one hundred. which the "Part" is the element you want to find in a formula. The "Whole" is the compound or the formula weight of the chemical formula, then times 100 because its a percent.
Ex: Find aluminium in Al₂(SO₄)₃

Formula Weight:    (2)(Al)-   (2)(26.9)=    53.8                                     53.8gAl
                                (3)(S)-    (3)(32.1)=    96.3          % composition-------------------- X100
                               (12)(O)- (12)(15.9)=  190.8                                   340.9gAl₂(SO₄)₃
                                                      Total= 340.9
% composition= 15.8%. there's 15.8% of Aluminium in the formula Al₂(SO₄)_3.