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Moles
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.
The 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,
 CnH2n 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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Copyright © 2008 Dr. Colin France. All Rights Reserved.