Percentage of Composition of a Compound

  1. To find the percentage of composition of a substance means to find the percentage of mass of each element in the molecule of the substance to the mass of the molecule.
  2. The percentage of mass of an element can be determined by the following equation:
Percentage of mass of an element = Sum of the Relative Atomic Mass of the Element  Relative Molecular Mass of the substance ×100%
Example
Calculate the percentage of composition of DDT (C14H9Cl5). [Relative atomic mass: Carbon = 14; Hydrogen = 1; Chlorine = 35.5]

Answer:
Relative molecular mass of DDT =14(12)+9(1)+5(35.5) =354.5
Percentage of carbon = 14(12) 354.5 ×100%=47.4%
Percentage of hydrogen = 9(1) 354.5 ×100%=2.5%
Percentage of chlorine = 5(35.5) 354.5 ×100%=50.0%