Empirical & Molecular Formulae
The molecular formula is the formula that shows the number and type of
each atom in a molecule
o 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
o 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 & Molecular Formulae Calculations
Empirical formula
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
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:
Elements Carbon Oxygen
Mass of each element
2.72 7.28
(g)
Atomic mass 12.0 16.0
Moles = mass / Ar = 0.227 = 0.455
Ratio (divide by smallest value) =1 =2
So, the empirical formula of the compound is CO2
The above example shows how to calculate empirical formula from the mass
of each element present in the compound
The example below shows how to calculate the empirical formula from
percentage composition
Worked Example
Calculating empirical formula from percentage
Determine the empirical formula of a hydrocarbon that contains 90.0% carbon and
10.0% hydrogen.
Answer:
Elements Carbon Hydrogen
Mass of each element
90.0 10.0
(g)
Atomic mass 12.0 1.0
Moles = mass / Ar = 7.5 = 10.0
Ratio (divide by smallest value) =1 = 1.33
Convert to whole number ratio
1x3=3 1.33 x 3 = 4
(x3 for this example)
So, the empirical formula of the compound is C3H4
Molecular formula
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
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:
Step 1: Calculate relative mass of the empirical formula
Relative empirical mass = (C x 4) + (H x 10) + (S x 1)
Relative empirical mass = (12 x 4) + (1 x 10) + (32 x 1)
Relative empirical mass = 90
Step 2: Divide relative formula mass of X by relative empirical mass
Ratio between Mr of X and the Mr of the empirical formula = 180/90
Ratio between Mr of X and the Mr of the empirical formula = 2
Step 3: Multiply each number of elements by 2
(C4 x 2) + (H10 x 2) + (S x 2) = (C8) + (H20) + (S2)
Molecular Formula of X is C8H20S2
Worked Example
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:
Elements Carbon Hydrogen Chlorine
Value (g or %) 24.2 4.1 71.7
Atomic mass 12.0 1.0 35.5
Moles = mass / Ar = 2.02 = 4.1 = 2.02
Ratio (divide by smallest) =1 =2 =1
So, the empirical formula of compound X is CH2Cl
The relative formula mass of the empirical formula is:
o Relative formula mass = (1 x C) + (2 x H) + (1 x Cl)
o Relative formula mass = (1 x 12.0) + (2 x 1.0) + (2 x 35.5)
o Relative formula mass = 49.5
Divide the relative formula mass of X by the relative formula mass of the
empirical formula
o Ratio between Mr of X and the Mr of the empirical formula = 99.0/45.9
o Ratio between Mr of X and the Mr of the empirical formula = 2
Multiply each number of elements by 2
o (C1 x 2) + (H2 x 2) + (Cl1 x 2) = (C2) + (H4) + (Cl2)
o The molecular formula of X is C2H4Cl2
