7 empirical and molecular formula

• What is the definition of empirical formula

  the simplest ratio of atoms of each element in a substance
• What are some examples of empirical formula

  • SiO2 (giant covalent) - the ratio of Si:O atoms in the lattice is 1:2
  • Al2O3 (ionic) - the ratio of Al³+:O²- ions in the lattice is 2:3
  • H2O (molecular) - the ratio of H:O atoms in a substance is 1:2
• Substance made of molecules also have a molecular formula. This indicates the number of atoms of each element in one molecule
• A)Finding the molecules formula from the formula mass and empirical formula

  E.g empirical formula =CH2 Mr = 42

  Formula mass of empirical formula = 14 therefore Mr ÷ Mr of empirical formula = 42÷ 14 = 3 therefore the molecular formula = 3× empirical formula = C3H6
• B)Finding the empirical formula of a compound from its composition by percentage or mass

  1. Write out the mass or percentage of each element
  2. Divide each mass or percentage by the Ar of the element (not the Mr)
  3. Find the simplest whole number ratio of these numbers by dividing the smallest number. If the values come out near 1/2 then times them by 2, if they are near 1/3 then times by 3

  E.g a compound is found to contain by mass iron 72.4% and oxygen 27.6%

  Fe = 72.4 ÷ 55.8 = 1.30

  O = 27.6 ÷ 16 = 1.73

  Simplest whole number ratio = 1.30:1.73 (divide by smallest I.e 1.30)

  Fe = 1.30 ÷ 1.30 = 1

  O = 1.73 ÷ 1.30 = 1.133

  1:1.133 ratio (involves 1/3 so ×3)

  3:4 ratio

  Therefore the empirical formula = Fe3O4
• Another example is

  0.25g of hydrogen reacts with oxygen to produce 4.25g of hydrogen peroxide (Mr= 34)

  Mass of oxygen reacting with hydrogen = 4.25 - 0.25 = 4.00g

  H = 0.25 ÷ 1 = 0.25

  O = 4 ÷ 16 = 0.25

  Ratio = 0.25:0.25

  1:1

  Empirical formula = HO

  Formula mass of empirical formula = 17.0 therefore Mr ÷ Mr of empirical formula = 34 ÷ 17 = 2

  Molecular formula = 2 × empirical formula = H2O2