Molecular, Empirical and Structural Formulae
Calculations of Formulae of Organic Compounds
The molecular formula of an organic compound is its actual formula. It gives the number of
each type of atom in a molecule, that is, the ratio of number of atoms in one molecule. This is
the same as the ratio of number of moles of atoms in one mole of the compound.
For example, the molecular formula of glucose is C6H12O6.
C:H:O
The ratio of moles:
is
6 : 12 : 6.
The empirical formula gives the simplest whole-number ratio of atoms in one molecule, or
the simplest whole-number ratio of moles of atoms in one mole of that compound. For
glucose this is CH2O (1 : 2 : 1). Therefore,
molecular formula = (empirical formula) x n
where n = 1,2,3, etc.
So, knowing the emprical formula and given the relative molecular mass of the compound, its
molecular formula can be deduced. The molecular formula often does not, however, give
information about the order in which the atoms within a molecule are bonded together. This is
provided by the structural formula. Two or more compounds which have the same
molecular formula but different structural formulae are called structural isomers. An
example is butane and methylpropane.
The empirical formula of a compound can be calculated if the masses or percentage masses
of each element are known.
For example, an organic acid, containing only the elements C, H and O, was shown on analysis to
contain C 40% and H 6.7%. Its relative molecular mass is 60, Identify the acid and draw its structural
formula.
That is, 100 g of the compound would consist of 40 g of carbon, 6.7 g of hydrogen, and 53.3 g of
oxygen. Therefore, simply divide these masses by the mass of one mole of each of the elements
respectively in order to find the ratio of number of moles of atoms.
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