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Moles

Calculating the Empirical Formula from a Reaction.

"Empirical" means "from experiment".

The empirical formula is the
simplest proportion of elements in a compound.
The empirical formula is always the same
as the actual formula for ionic compounds.

For molecular compounds,
the empirical formula is often (but not always)
the same as the molecular formula.

What is a Molecular Formula?

A molecular formula is the actual proportion
of elements in a molecular compound.
CO₂, NH₃, C₂H₄, and C₃H₆
are examples of molecular formulae.

For an ionic compound, a reaction might show
that the proportion of calcium to oxygen is 8 to 8.
The empirical formula reduces the proportion
to the lowest whole numbers (divide by 8).
So, the empirical formula is CaO,  not Ca₈O₈.

Similarly, a reaction might show
that the proportion of sodium to carbon to oxygen
is 6 to 3 to 9 (divide by 3).
The empirical formula is Na₂CO₃,  not Na₆C₃O₉.

For a molecular compound, a reaction might show that
the proportion of carbon to hydrogen is 3 to 6 (divide by 3).
The empirical formula is CH₂,  not C₃H₆.
CH₂ does not exist as a molecule.
The molecular formula could be any multiple of CH₂,
C_nH_2n where n is a whole number.
The molecule is an alkene, but to know which
particular alkene it is, you must
also know the relative molecular mass (RMM).

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