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EMPIRICAL FORMULA – gives the chemical symbols of the atoms present in a compound and implies the SIMPLEST RATIO between the numbers of these atoms by integer sub-scripts in the written formula.

To determine the empirical formula of a compound, the chemical amount of each element present in
a sample must be calculated.  Thence we determine the simplest whole-number ratio of the chemical amounts. Effectively, two or more  n = m / M  type calculations are being carried out side by side.

 11.2  CALCULATING EMPIRICAL FORMULAE FROM MASS % COMPOSITIONS 

It is convenient to set the results out in tabular form. In the following Worked Example, however, the procedure in establishing the table is taken step by step.

WORKED EXAMPLE 10

Solution

A hydrocarbon contains by mass 80.0 % of carbon. Calculate its empirical formula.

[ Ar(H) = 1.01;  Ar(C) = 12.01 ]

Taking the basis of the calculation as 100 g of sample - sensible since data are mass % - then the mass of each constituent should be obvious: C 80.0 g; H 20.0 g.

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Now you can try a similar problem.

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Courtesy of the Faculty of Science (Chemistry) at The University of Sydney (AU), the link below provides an easy-to-use calculator that enables you to check any empirical formula calculations carried out.

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