1. Quantitative Chemistry Part 2
Empirical Formula
and
Molecular Formula

2. Empirical Formula What is meant by the term ‘empirical’?
The term ‘empirical’ means ‘determined by experiment’
What does the empirical formula tell us?
The empirical formula tells us the simplest formula of a compound – and just shows the ratio of the numbers of atoms each element in a compound.

3. Example
The chemical formula (molecular formula) for hydrogen peroxide is H2O2
The elements hydrogen and oxygen are present in the ratio, 2:2 or simply, 1:1
Hence, the empirical formula for hydrogen peroxide is: HO
In other words, chemical analysis of a sample of hydrogen peroxide would show that hydrogen and oxygen atoms were present in the ratio 1:1

4. Another example
Butane is the gas used as a propellant in some aerosol cans.
Its chemical (or molecular formula) is C4H10
The elements carbon and hydrogen are present in the simple ratio, 2:5
Therefore, the empirical formula for butane is:
C2H5

5. Calculation of Empirical Formula from experimental data.
A compound has the following composition by mass.
Na = 29.1%; S = 40.5%; O = 30.4%
Hence, in a 100g sample, there would be:
29.1g of Na; 40.5g of S; and 30.4g of O
The relative atomic masses of these elements are:
Na = 23; S = 32; O = 16.

6. …calculation continued
Atoms of sodium = = =
Mass Ar
29.1 23
1.265
Atoms of sulphur = = =
Mass Ar
40.5 32
1.266
Atoms of oxygen = = =
Mass Ar
30.4 16
1.900
For the empirical formula, the numbers of atoms of each element must be whole numbers (integers!)

7. …..continued The ‘trick’ to obtain whole numbers is to:
divide each of the previous answers by the smallest!
i.e
1.265
 Atoms of sodium =
= 1.00
1.266
1.265
 Atoms of sulphur =
= 1.00
1.900
1.265
 Atoms of oxygen =
= 1.50

8. …….continued If we still do not have whole numbers:
Multiply all answers by a suitable small number – i.e. 2, 3 or 4
In this case, multiply all answers by 2, so that the empirical formula becomes:
Na2S2O3

9. Molecular Formula
The molecular formula for a compound is the formula that shows:
the actual numbers of each type of atom in the molecule
To calculate the molecular formula of a compound, we need to know its empirical formula and its molecular (formula ) mass.

10. Example
What is the molecular formula of a compound which has the empirical formula, C1H1 and a molecular mass (Mr) of 78?
C = 12; H = 1.
We start by saying : (C1H1)n = 78
 (12 + 1)n = 78
 13n = 78
 n = 78 13
Hence, the molecular formula is C6H6
= 6

11. tyui
tyui
tyui