molecular and empirical formula

• what is the empirical formula?

  simplest whole number ratio of each atom of an element in a molecule
• what is the molecular formula?

  actual number of atoms of each element in a molecule
• example: a compound has the following percentage composition by mass; C - 40% H - 6.6% O - 53.4% determine the empirical formula

  elements C H O

  % by mass 40 6.6 53.4

  divide by Ar 3.3 6.67 3.33

  divide by 1 2 1

  smallest value

  empirical formula - CH2O
• example: 3.53g of Fe reacts with Cl to form 10.24g of FeCl3, find the empirical and molecular formula. Mr is 1055.1

  2Fe + 3Cl2 --> 2FeCl3

  mass of Cl - 3.53 / 55.8 x 3/2 x 71 = 6.74g

  moles x ratio x Mr

  Fe Cl FeCl3

  3.53 6.74 10.24

  0.063 0.189 0.112

  1 3 1.75

  4 12 7

  Fe7Cl21