EMPIRICAL
FORMULAS
Methanal vs. acetic acid
Both substances have C:H:O in a 1:2:1 ratio,
but they are completely different
substances.
Methanal vs. acetic acid
The EMPIRICAL FORMULA for both these
substances is CH2O.
The MOLECULAR FORMULA for acetic acid
is C2H4O2.
EMPIRICAL VS.
MOLECULAR FORMULAS
EMPIRICAL FORMULAS show the simplest
whole number ratio of elements in a
compound.
MOLECULAR FORMULAS show the element
symbols and the exact no. of each type of
atom in a molecular compound.
**formulas for ionic compounds are ALWAYS
empirical.
Comparing Empirical &
Molecular Formulas
• Sometime the empirical and molecular
formulas are the same and sometimes they
are different
Glucose
Molecular
C6H12O6
Formula
Empirical
Formula
Hydrogen
peroxide
Hydrazine
Water
H2O2
N2H4
H2O
Finding the Empirical Formula from
Percent Composition
1.Use a chart to organize your work\
Atom
Assume 100g:
Mass (g)
MM (g/mol)
n (mol)
Smallest
Ratio
Whole
Number
A rhyme to help you
remember what goes in
the chart:
Percent to mass
Mass to mole
Divide by small
Multiply til whole
1.Assume you have 100 g sample so that your
mass % is the same as your mass in grams.
To get a whole number at the end…multiply.
2.
If the decimal ends in….
...33
…25
... 1.5
…,67
….75
Fraction
1/3
1/4
½
Multiply by…
3
4
2
EXAMPLE 1: Find the empirical formula of
a compound with 35.4% sodium and the
remainder nitrogen.
STEP 1: Always assume you are working with
100 g of the compound.
Atom
Assume
100g:
Mass (g)
MM (g/mol)
Na
35.4g
22.99
N
64.6g
14.01
n (mol)
Smallest
Ratio
Whole
Number
EXAMPLE 1: Find the empirical formula of
a compound with 35.4% sodium and the
remainder nitrogen.
STEP 2: Calculate the amount of each element
in moles using n=m/MM
Atom
Assume 100g:
Mass (g)
MM (g/mol)
n (mol)
Na
35.4g
22.99
n=m/MM
=35.4/22.99
=1.539…
N
64.6g
14.01
n=4.61…
Smallest
Ratio
Whole
Number
EXAMPLE 1: Find the empirical formula of
a compound with 35.4% sodium and the
remainder nitrogen.
STEP 3: Determine the whole number ratio by
dividing each element mole value by the
lowest number.
Atom
Assume 100g:
Mass (g)
MM (g/mol)
n (mol)
Na
35.4g
22.99
n=m/MM
=35.4/22.99
=1.539…
N
64.6g
14.01
Smallest
Ratio
1
n=4.61…
3
Whole
Number
EXAMPLE 1: Find the empirical formula of
a compound with 35.4% sodium and the
remainder nitrogen.
STEP 4: Multiply each number by an integer to
obtain all whole numbers.
Atom
Assume 100g:
Mass (g)
MM
(g/mol)
Na
35.4g
22.99 n=m/MM
=35.4/22.99
=1.539…
N
64.6g
n (mol)
Smallest
Ratio
Whole
Number
1
14.01 n=4.61…
3
Empirical Formula is NaN3**not needed this
time
EXAMPLE 2: Determine the empirical
formula of a compound that contains 69.9%
iron and 30.1% oxygen by mass.
***Assume 100g of substance***
STEP 1: Calculate the amount of each element
in 100g.
EXAMPLE 2: Determine the empirical
formula of a compound that contains 69.9%
iron and 30.1% oxygen by mass.
STEP 2: Divide the amount of each element by
the smallest amount.
EXAMPLE 2: Determine the empirical
formula of a compound that contains 69.9%
iron and 30.1% oxygen by mass.
STEP 3: Find an equivalent whole number ratio
and write the empirical formula.
Tips for Solving Empirical
Formula Problems
• Don’t round until the very end.
•
If the value is 0.95 round to 1, if
the value is 0.05 round to 0, if
between 0.45 to 0.55 round to
0.5.
Determining the Empirical
Formula by Experiment
•
•
You can calculate a molecular formula
using the periodic table, for example
you know that pure copper will oxidize
to become CuO. But, as a scientist
you need to confirm this by
experimentation.
This can be done using a variety of
methods such as a synthesis reaction.
HOMEWORK:
pp. 292 #1 (practice)
pp. 293 #1-5, 7
Molecular Formula
• The molecular formula (actual formula) can
only be found if you have both the:
o Molar mass of the compound &
o Empirical Formula (or percent composition)
Steps to Finding
*the molar mass Molecular
Formula
AND
*Empirical formula (or percent composition) 1.Find the molar mass of the empirical formula
2.Find the ratio of the molar mass of the
Finding Molecular Formula
The molecular formula (actual formula) for a compound can only be found if you know
1) Find the molar mass of the empirical formula. 2) Use the following formula to find the ratio of the molar masses of the compound and empirical formula.
compound to the empirical formula using:
Ratio = Mcompound Mempirical formula
3. Multiply the empirical formula by the ratio to
3) Multiply the empirical formula subscripts by the ratio to obtain the molecular formula.
obtain the molecular formula.
ex. The empirical formula for Vitamin C is C3H4O3 and its molar mass is 176.17 g/mol. Find the molecular formula.
Example 1
• The empirical formula for Vitamin C is
C5H4O3 and its molar mass is 176.17 g/mol.
Find the molecular formula
• MMEF = 88.07 g/mol
• Ratio = MMMF = 176.17g/mol
•
=2
MMEF
88.07g/mol
Molecular Formula = C5 x 2 H4 x 2 O3 x 2
= C10H8O6
Learning Check
A compound with an empirical formula of
C2OH4 and a molar mass of 88g/mol. What is
the molecular formula of the compound?
C4O2H8
Exit Card
• A sample of indium chloride with a mass of
0.5000g is found to have 0.2404g of chlorine.
What is the empirical formula of the indium
compound?