Empirical Formula
The simplest formula that
represents the whole
number ratio between the
elements in a compound.
Empirical Formula
Reduce the subscripts in a
formula to their simplest
whole number ratio.
Empirical Formula
The molecular formula for
hydrogen peroxide is H2O2,
its empirical formula is HO.
Empirical Formula
Subscripts are
usually thought of
as a ratio of atoms.
Empirical Formula
Subscripts can also be
thought of as a ratio of
moles of atoms.
Empirical Formula
To determine an
empirical formula,
one must determine
the mole ratio.
Moles
A mole is the number of
atoms needed to equal
the atomic mass of
an element.
Moles
The atomic mass of
an element in grams
is equal to one mole
of its atoms.
Moles
One mole of anything
23
is equal to 6.02 X 10
Sample Problem
Determine the empirical
formula of a compound that
is composed of 36.5% Na,
25.4% S, and 38.1% O.
HINT:
You can count on the
elements being given
in the problem in the
same order they are
found in the formula.
Step 1
Find the number
of moles of each
element present.
Step 1
Since the amount of each
element is given in
percentage, you must convert
the percentage to a mass...
Step 1
If 100 grams of the sample are
assumed, the percentages
given are the same as grams.
Step 1
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Step 2
Determine the whole
number mole ratio.
Step 2
QuickTime™ and a
TIFF (Uncompress ed) dec ompres sor
are needed to s ee this pic ture.
Step 3
Use the mole ratio for
the subscripts of each
element in the formula.
Step 3
The empirical formula is
Na2SO3
Finding a molecular formula
from an empirical formula
The formula mass of the sample
problem is 126 u. If the
molecular mass is known to be
378 u, what is the molecular
formula?
Solution
Divide the molecular mass
by the empirical formula
mass to get a whole
number.
378
=3
126
Solution
Multiply each subscript
in the empirical formula
by this whole number.
Solution
You now have the
molecular formula -
Na6S3O9
Hydrated Crystals