The empirical formula is the simplest chemical formula that shows the lowest whole number ratio of the elements in the compound. This is different from the molecular formula, which has the number of atoms of each element that make up that molecule. The empirical formula may or may not be the actual molecular formula for that compound. It is also possible for different compounds to have the same empirical formula.
For example, benzene (C6H6) and ethyne (C2H2) both have the same empirical formula of CH. It is important to note that the empirical formula does not describe the composition of the molecule. Many compounds tend to have the same empirical formulas as their molecular formulas.
For example, the empirical formula and molecular formula for H2O and CO2 are the same.
Example:
If a compound has a percent composition of 85.6% carbon and 14.4% hydrogen, what is the empirical formula?
1) Assume the sample is 100 g.
Mass of carbon = 85.6 g
Mass of hydrogen = 14.4 g
2) Convert the mass of carbon and hydrogen into moles.
Moles of C = (85.6 g) / (12.01 g/mol)
= 7.127 moles
Moles of H = (14.4 g) / (1.01 g/mol)
= 14.257 moles
3) Divide the molar values by the lowest molar value.
C = 7.127 moles / 7.127 moles
= 1
H = 14.257 moles / 7.127 moles
= 2.0004
Thus, the empirical formula is CH2.
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