Chung Cheng High School (Main)
E-learning on Empirical Formula and Molecular Formula
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)
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Types of formulae:

Empirical Formula : simplest whole number ratio of the atoms present

Molecular Formula : shows the actual number and kinds of atoms present

Structural Formula: shows how the atoms are joined in the molecule.
For example, the molecular formula of ethane is C2H6 while its empirical formula is CH3.
The structural formula is shown above, alongside a ball-and-stick model of ethane.
Worked examples: Finding Empirical Formula
1. Find the empirical formula of a compound containing 1.20g of Mg and 0.80g of O.
Element
Mg
O
Mass (g)
1.20
0.80
Molar mass (g/mol)
24
16
Number of moles
Ratio
1.20 ÷ 24 = 0.05 0.80 ÷ 16 = 0.05
0.05 ÷ 0.05 = 1
0.05 ÷ 0.05 = 1
Note:
 Present answers in a table.
 Show units for mass in grams
and molar mass in g/mol.
 Workings for number of moles
and the ratio of the elements.
Therefore, the empirical formula of magnesium oxide is MgO.
2. Find the empirical formula of a compound consisting 2.80g of iron combined with 1.20g of oxygen.
Element
Fe
O
Mass (g)
2.80
1.20
Molar mass (g/mol)
56
16
Number of moles
2.80 ÷ 56 = 0.05
1.20 ÷ 16 = 0.075
Ratio
0.05 ÷ 0.05 = 1
0.075 ÷ 0.05 = 1.5
Simplest whole number ratio
2
3
Therefore, the empirical formula of the compound is Fe2O3.
3. An oxide of sulfur contains 40% sulfur and 60% oxygen by mass. Find the empirical formula.
Assuming 100g of the sample,
Element
S
O
Mass (g)
40
60
Molar mass (g/mol)
32
16
Number of moles
40 ÷ 32 = 1.25
60 ÷ 16 = 3.75
Ratio
1.25 ÷ 1.25 = 1 3.75 ÷ 1.25 = 3
Therefore, the empirical formula of the compound is SO3.
Worked examples: Finding Molecular Formula
4. The empirical formula of ethane is CH3. Given that the relative molecular mass of ethane is 30, what is
the molecular formula?
Let the molecular formula of ethane be CnH3n
Mr of ethane = (n x 12) + (3n x 1) = 15n
I.e.
15 n
= 30
n
= 30 ÷ 15
=2
Hence, the molecular formula of ethane is C2H6
5. The empirical formula of a compound is H2CO2. Given its relative molecular mass is 46, find the
molecular formula.
Let the molecular formula of the compound be H2nCnO2n.
Mr of the compound = (2n x 1) + (n x 12) + (2n x 16) = 46n
i.e.
46 n
= 46
n
= 46 ÷ 46
=1
Hence, the molecular formula of the compound is H2CO2
Practice Questions
Do the following questions, showing workings clearly for each step.
1. Find empirical formula of the compound which has 43.4% sodium, 11.3% carbon and 45.3% oxygen.
(Ans: Na2CO3)
2. A compound of nitrogen and hydrogen has a mass of 68 g and contains 56 g of nitrogen.
a. What is the mass of hydrogen in the compound?
b. Calculate the empirical formula of the compound.
(Ans: NH3)
c. If the relative molecular mass of the compound is 34, what is its molecular formula? (Ans: N2H6)