Molecular Formulas
A molecular formula is either the same as an empirical formula or it is a simple whole number multiple of
its empirical formula.
Determining Molecular Formulas
Calculate the molecular formula of a compound whose molar mass is 60.0 g/mol and has an empirical formula of CH4N.
What you need:
Empirical Formula
Molar Mass
Calculate the molar mass of the empirical formula
C: 12.0g x 1
H: 1.0g x 4
N: 14.0g x 1
= 12.0g
= 4.0g
= 14.0g
30.0 g/mol
Molecular Formula Molar Mass
= Multiple
60.0 g/mol
Empirical Formula Molar Mass
30.0 g/mol
Empirical Formula X Multiple = Molecular Formula
CH4N x 2
=2
=
C2H8N2
Determining Molecular Formulas
Find the molecular formula of ethylene glycol which has a molar mass of 62 g/mol and an empirical formula of CH3O.
What you need:
Empirical Formula
Molar Mass
Calculate the molar mass of the empirical formula
C: 12.0g x 1
H: 1.0g x 3
O: 16.0g x 1
= 12.0g
= 4.0g
= 16.0g
31.0 g/mol
Molecular Formula Molar Mass
= Multiple
62 g/mol
Empirical Formula Molar Mass
31.0 g/mol
Empirical Formula X Multiple = Molecular Formula
CH3O x 2
=2
=
C2H6O2
Determining Molecular Formulas
The compound methyl butanoate has a percent composition of 58.8% C, 9.8% H, and 31.4% O and has a molar mass
of 102 g/mol. What is its molecular formula?
What you need:
Empirical Formula
Empirical Formula = C5H10O2
Molar Mass
58.8g C
Calculate the molar mass of the
empirical formula
1 mol C
12.0g C
9.8g H
1 mol H
1.0g H
31.4g O
1 mol O
16.0g O
= 4.90 mol C
= 2.50
x2
=5
1.96 mol
= 9.8 mol H
= 5.0
x2
C: 12.0g x 5
H: 1.0g x 10
O: 16.0g x 2
= 60.0g
= 10.0g
= 32.0g
102.0 g/mol
= 10
1.96 mol
102 g/mol
=1
102.0 g/mol
= 1.96 mol O
= 1.00
1.96 mol
x2
=2
C5H10O2 x 1
=
C5H10O2
Determining Molecular Formulas
What is the molecular formula of a compound that has a percent composition of 94.1% O and 5.9% H and has a
molar mass of 34 g/mol.
What you need:
Empirical Formula
Empirical Formula = HO or OH
Molar Mass
5.9g H
Calculate the molar mass of the
empirical formula
1 mol H
1.0g H
94.1g O
1 mol C
16.0g O
H: 1.0g
O: 16.0g
= 5.9 mol H
= 1.0
5.88 mol
17.0 g/mol
= 5.88 mol O
= 1.00
34 g/mol
5.88 mol
17.0 g/mol
HO x 2
=
H2O2 or O2H2
=2