Percentage Composition

Percent partwhole X 100% mass of element mass of compound X 100% X 100% percent by mass =

Percent Composition start with chemical formula start with chemical formula assume 1 moleassume 1 mole divide mass each element by formula mass (FM) of compound divide mass each element by formula mass (FM) of compound

Percent Composition of H 2 O 2 H’s = 2 x 1.0 g = 2.0 g 1 O = 1 x 16.0 g = 16.0 g molar mass = 2.0 g g = 18.0g/mol % H = 2.0g x 100% = 11.11% 18.0g 18.0g % O = 16.0g x 100% = 88.89% 18.0g 18.0g

Percent Composition of C 6 H 12 O 6 6 C’s = 6 x 12g = 72g 12 H’s = 12 x 1g = 12g 6 O’s = 6 x 16g = 96g molar mass = grams/mole % C = % H = % O = (72g/180g) X 100% = 40.00% (12g/180g) X 100% = 6.67% (96g/180g) X 100% = 53.33% 100%

Percentage Composition remember to calculate FM! remember to calculate FM! nowhere in word problem will it tell you that!nowhere in word problem will it tell you that! sum of individual element %’s must add up to exactly 100%sum of individual element %’s must add up to exactly 100% 2 decimal places usually works 2 decimal places usually works