Transcript Quantitative Chemistry Part 2

Quantitative Chemistry
Part 2
Empirical Formula
and
Molecular Formula
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.
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
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
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.
…calculation continued
Atoms of sodium =
Mass
Ar
Atoms of sulphur =
Mass
Ar
Atoms of oxygen =
Mass
Ar
=
29.1
23
= 1.265
=
40.5
32
= 1.266
=
30.4
16
= 1.900
For the empirical formula, the numbers of atoms of
each element must be whole numbers (integers!)
…..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 =
1.265
1.265
= 1.00
 Atoms of sulphur =
1.266
1.265
= 1.00
 Atoms of oxygen =
1.900
1.265
= 1.50
…….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
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.
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
78
n=
=6
13
Hence, the molecular
formula is C6H6
• tyui
• tyui
• tyui