Empirical Formula
The empirical formula indicates the ratio
of the atoms of an element in a compound
Empirical Formula
 The molecular formula of a compound
indicates how many moles of each element
are in 1 mole of the compound.
 For example: Glucose is C6H12O6. How many
moles of carbon are in 1 mole of glucose?
CORRECT
 There are 6 moles of carbon in 1 mole of
glucose because of the subscript 6 after the C
in the formula.
 Now look at the ratio of the atoms in glucose.
Glucose is C6H12O6.
The ratio of atom is C1H2O1.
Therefore the empirical formula is CH2O.
PRACTICE
 What is the empirical formula of C6H12?
 CH2
 What is the empirical formula of C6H6?
 CH
Empirical Formula
We will now learn how to calculate an empirical
formula from experimental data.
For example: We have a compound that has
70% iron and 30% oxygen.
It is easiest to assume we have a 100g sample
of the compound. Let’s begin.
Step #1
 Calculate the number of grams of each
element in 100g of the compound.
 70% iron is 70g of iron
 30% oxygen is 30g of oxygen
Step #2 – Convert grams to moles
Convert grams to moles
70g Fe 1 mole Fe
55.8g Fe
30g O
1 mole O
16.0 g O
1.25 moles Fe
1.87 moles O
Fe1.25O1.87 – That’s strange looking
 Our last calculation showed that the ratio of
the elements in the compound is 1.25 moles
of iron to 1.87 moles of oxygen.
 Chemical formulas use whole numbers to
show the ratio of the elements in a compound
 Our next step is to convert to the lowest
whole number ratio for the formula.
Step #3
 Divide each number by the smallest number.
1.25 moles Fe
1 moles Fe
1.25
1.87 moles O
1.25
1.5 moles O
Problem?
 Our results did not give us a whole number
ratio.
 Most of the time, step #3 will yield whole
number ratios, but occaisionally you will have
to go on to step #4.
Step #4
 If necessary, multiply by a factor so that all of
the numbers are whole numbers.
 Since the mole ratio in the problem is 1:1.5,
multiply by numbers by 2.
1 mole Fe x 2 = 2 moles Fe
1.5 moles O x 2 = 3 moles O
Final Answer: Fe2O3
 Congratulations, you can determine an
empirical formula from experimental data.
 Now, let’s learn how to determine the
MOLECULAR FORMULA from the empirical
formula.
 Remember, the empirical formula is the
lowest whole number ratio, the MOLECULAR
FORMULA is the actual formula of the
compound
Determine the Molecular Formula
Example Problem:
Given a compound with an empirical formula
of CH2 and a molecular mass of 42, what is
the Molecular Formula?
Step #1
 Calculate the mass of the empirical formula.
CH2 is composed of one carbon atom and two
hydrogen atoms. Therefore the molecular
mass is 14.0g.
Step #2
Divide the molecular mass by the empirical
mass.
42g
14g
3
This is the mass ratio.
Step #3
 Multiply the subscripts of the empirical
formula by the mass ratio.
CH2 X 3 = C3H6
The molecular formula is C3H6