Empirical & Molecular Formulae

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

The 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

Calculating empirical formula from mass

Determine the empirical formula of a compound that contains 10 g of hydrogen and 80 g of oxygen.

Answer:

Elements	Hydrogen	Oxygen
Mass of each element (g)	10	80
Atomic mass	1.0	16.0
Moles = mass / A_r	$\frac{10}{1.0}$ = 10	$\frac{80}{16.0}$ = 5
Ratio (divide by smallest value)	$\frac{10}{5}$ = 2	$\frac{5}{5}$ = 1

Calculating empirical formula from %

Determine the empirical formula of a compound that contains 85.7% carbon and 14.3% hydrogen.

Answer:

Elements	Carbon	Hydrogen
Mass of each element (%)	85.7	14.3
Atomic mass	12.0	1.0
Moles = mass / A_r	$\frac{85.7}{12.0}$ = 7.14	$\frac{14.3}{1.0}$ = 14.3
Ratio (divide by smallest value)	$\frac{7.14}{7.14}$ = 1	$\frac{14.3}{7.14}$ = 2.00

The molecular formula gives the exact number 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

Calculating molecular formula

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

What is the molecular formula of X?

Relative atomic mass Carbon: 12 Hydrogen:1 Sulfur:32

Answer:

Step 1: Calculate the relative formula mass of the empirical formula
- Relative formula mass = (C x 4) + (H x 10) + (S x 1)
- Relative formula mass = (12 x 4) + (1 x 10) + (32 x 1)
- Relative formula mass = 90
Step 2: Divide the relative formula mass of X by the relative formula mass of the empirical formula
- Ratio between M_r of X and the M_rof the empirical formula = 180/90
- Ratio between M_r of X and the M_rof the empirical formula = 2
Step 3: Multiply each number of elements by 2
- (C_{4 x 2}) + (H_{10 x 2}) + (S_{1 x 2}) = (C₈) + (H₂₀) + (S₂)
- The molecular formula of X is C₈H₂₀S₂