Empirical formulas are the simplest formula of a compound that show only the simplest ratios.
-Not the actual atoms
Ex: - The empirical formula for Hydrogen gas is H
- Dinitrogen TetraOxide is not N2O4 in an empirical formula format. It is NO2.
Molecular formulas give the actual number of atoms.
We need to know the ratio of each element to determine the empirical formula.
This table will help to determine the ratio for an element.
Atom Mass Molar Mass Moles Smallest Mole Ratio
C 8.4g 12.0g 0.7/0.35 = 2 2
H 2.1g 1.0g 2.1/0.35 = 6 6
O 5.6g 16.0g 0.35/0.35 = 1 1
The simplest ratio could be decimals.
For certain decimals you may need to multiply everything by a common number.
Decimal Multiplying Coefficient
0.5 2
0.33/0.66 3
0.25/0.75 4
0.2, 0.4, 0.6, 0.8 5
Atom Mass Molar Mass Moles Smallest Mole Ratio
C 50.5g 12.0g 4.21/3.16 = 1.32 x 3 4
H 5.26g 1.0g 5.26/3.16 = 1.66 x 3 5
N 44.2g 14.0g 3.16/3.16 = 1 x 3 3
To find the molecular formula you need the molar mass. If you know the empirical formula.
Empirical Molecular
C2H6O ?
? 138g/mol
12.0(2)+1.0(6)+16.0= 46.0g/mol
Empirical Molecular
C2H6O ?
46.0g/mol 138g/mol
138.0/46.0= 3 (C2H6O)3
Empirical Molecular
C2H6O C6H18O3
46.0g/mol 138.0g/mol
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