Download 6.9 MOLECULAR FORMULAS

6.9 MOLECULAR FORMULAS
The knowledge of a molecular formula as well as an empirical formula comes into question in
many scenarios. Forensics is one such area, where the knowledge of the types and amount of
elements in a compound is essential.
A suspect in an arson investigation also happens to be a scientist in a chemical refinery and
works with many hydrocarbons. Investigators find samples of a substance at the crime scene
and when sent to the Forensics Lab, it is revealed that the chemical has an empirical formula of
CH2. There are many compounds with the empirical formula CH (below). Analyzing the physical
and chemical properties of the substance and looking at its molar mass would help in
determining exactly what the substance is. We can use mass spectrometry to determine the
molar mass of the compound.
Table 1: Data for Identifying 3 Organic Compounds with the Empirical Formula CH
Name
Molecular Whole # Molar Mass
Function
Formula
Multiple (g/mol)
Ethylene
C2H4
2
28.06
Butene
Cyclohexane
C4H8
C6H12
4
6
56.12
84.18
Natural plant hormone important in ripening,
historically used as an anaesthetic
Product of crude-oil refining
Precursor in the synthesis of nylon
Molecular Formula Subscripts = n x Empirical formula subscripts (n = 1,2,3…)
Molar Mass of a Compound = n x molar mass of an empirical formula (n = 1,2,3…)
The molecular formula shows the actual number of atoms of each element in a compound.
FINDING A MOLECULAR FORMULA GIVEN ITS EMPIRICAL FORMULA
Example 1: Determine the molecular formula of a compound with empirical formula CH 2 and
molar mass 84.18 g/mol.
MCH2 = MC + 2MH
(12.01 g/mol) + 2(1.01 g/mol)
(12.01 g/mol) + (2.02 g/mol)
14.03 g/mol
n = 84.18 g/mol
14.03 g/mol
n = 6.00
Therefore, the molar mass of the compound is 6x the molar mass of the empirical formula. We
can multiply the subscripts in the empirical formula by 6.
Therefore, the molecular formula is C6H12.
FINDING A MOLECULAR FORMULA WITH PERCENTAGE COMPOSITION AND MOLAR MASS DATA
Example 2: An analysis shows that a compound is made of 92.3 % C and 7.70 % H. The molar mass
of this compound is 78 g. What is the molecular formula?
C
92.3 %
92.3 g/12.0 g/mol = 7.69 mol/ 7.69 mol = 1
H
7.70 %
7.70 g/1.00 g/mol = 7.70 mol/ 7.69 mol = 1
Therefore, the empirical formula is CH. We know for sure that it is a hydrocarbon.
The molar mass of the empirical formula is:
1(12.01 g/mol) + 1(1.01g/mol) = 13.02 g/mol
MF = n x (EF)
13.02 g/mol x n = 78 g/mol
n = 78 g/mol
13.02 g/mol
n=6
Therefore the molecular formula is C6H6.