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Section 1: Physical Chemistry 1
2: Amount of substance
2.4 Empirical & molecular formulae
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Cards (6)
Empirical formula
- the
simplest
whole
number
ratio
of
atoms
of each element in the compound
Molecular formula
- shows the
actual
number
of
atoms
of each element in a molecule
Calculating empirical formula from masses
Calculate the number of
moles
of each
element
Divide
each of the numbers of moles by the
smallest
of these numbers to find the
ratio
.
Calculating empirical formula from percentage compositions
Calculate the number of
moles
that would be present in 100g (
divide
by
Mr
)
Divide
each of the numbers of moles by the
smallest
of these numbers to give the
ratio
Converting empirical formulae into molecular formulae
Divide
the
Mr
of the
compound
by the Mr of the
empirical
formula
Hydrated salts contain
waters
of
crystallisation.
This means that water molecules are
present
inside the crystals and therefore part of the
structure.
These water molecules are therefore
included
in the formulae for those salts.
