Empirical and Molecular Formula

Empirical and Molecular Formula

  • The molecular formula is the formula that shows the number and type of each atom in a molecule.

    • E.g. the molecular formula of ethanoic acid is C2H4O2

  • The empirical formula is the simplest whole number ratio of the elements present in one molecule or formula unit of the compound.

    • E.g. the empirical formula of ethanoic acid is CH2O

  • Organic molecules often have different empirical and molecular formulae.

  • Simple inorganic molecules however have often similar empirical and molecular formulae.

  • Ionic compounds always have similar empirical and molecular formulae.

Empirical Formula Calculations

  • Empirical formula is the simplest whole number ratio of the elements present in one molecule or formula unit of the compound.

  • It is calculated from knowledge of the ratio of masses of each element in the compound.

  • The empirical formula can be found by determining the mass of each element present in a sample of the compound.

  • It can also be deduced from data that gives the percentage compositions by mass of the elements in a compound.

  • Worked Example One:

    • Calculating empirical formula from mass

      Determine the empirical formula of a compound that contains 2.72 g of carbon and 7.28 g of oxygen.

  • Answer:

    • So, the empirical formula of the compound is CO2

  • Worked Example Two:

    • Calculating empirical formula from percentage

      Determine the empirical formula of a hydrocarbon that contains 90.0% carbon and 10.0% hydrogen.

  • Answer:

    • So, the empirical formula of the compound is  C3H4 

Molecular Formula Calculations

  • The molecular formula gives the exact numbers of atoms of each element present in the formula of the compound.

  • The molecular formula can be found by dividing the relative formula mass of the molecular formula by the relative formula mass of the empirical formula.

  • Multiply the number of each element present in the empirical formula by this number to find the molecular formula.

  • Worked Example One:

    • Calculating molecular formula

    • The empirical formula of X is C4H10S and the relative molecular mass of X is 180

               What is the molecular formula of X?

               (Ar data: C = 12, H = 1, S = 32)

  • Answer:

    • So the Molecular Formula of X is C8H20S2

  • Worked example Two:

    • Calculating empirical formula and molecular formula

    • Analysis of a compound X shows that it contains 24.2 % by mass of carbon, 4.1 % by mass of hydrogen and 71.7% by mass of chlorine.

    • Calculate the empirical formula of X.

    • Use this empirical formula and the relative molecular mass of X (Mr = 99.0) to calculate the molecular formula of X.

  • Answer:

    • So, the empirical formula of compound X is CH2Cl

    • The relative formula mass of the empirical formula is:

      • Relative formula mass = (1 x C) + (2 x H) + (1 x Cl)

      • Relative formula mass = (1 x 12.0) + (2 x 1.0) + (1 x 35.5)

      • Relative formula mass = 49.5

    • Divide the relative formula mass of X by the relative formula mass of the empirical formula

      • Ratio between Mr of X and the Mr of the empirical formula = 99.0/45.9

      • Ratio between Mr of X and the Mr of the empirical formula = 2

    • Multiply each number of elements by 2

      • (C1 x 2) + (H2 x 2) + (Cl1 x 2) = (C2) + (H4) + (Cl2)

      • The molecular formula of X is C2H4Cl2