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Empirical and Molecular Formula: Describing in different ways
Every formula which expresses a law of nature is a hymn of praise to God.
-Maria Mitchell
OBJECTIVES: At the end of the module, the students will be able to:
a. solve for the percentage composition of compounds.
b. determine the empirical formula of compounds.
c. determine the molecular formula of compounds.
CONCEPT INTRODUCTION:
Recently discovered compounds or suspected substances found in a crime scene
are usually analyzed experimentally by chemists. They determine the components of a
chemical compound by analyzing composition data and percentage by mass of the
sample and translate these percentages into mole ratios. Finally, they determine the
simplest formula of the compound. The simplest formula, or empirical formula, consists
of the chemical symbol of the elements with the smallest whole-number ratio of the
atoms as subscripts.
Name: ___________________________
Date: ______________
PERCENTAGE COMPOSITION
The law of definite proportion states that compounds contain exact proportions
of each element by mass, regardless of the method of preparation. The sum of all of
the atomic masses of elements in a formula is called the formula mass. If it is
expressed in grams, then it is called a gram formula mass or molar mass. If it
represents the sum of the masses of all of the elements in a molecule then it is called a
molecular mass. To find the percentage of each element in a compound, it is necessary
to compare the total mass of each element with the formula mass.
Example
Calculate the mass of each element in sodium carbonate, Na2CO3 .
Step 1 : Calculate the formula mass for K2CO3 . Find the atomic mass of each
element from the periodic table. Multiply it by the number of times it appears
in the formula and add up the total
2sodium atoms
Na
2 x 22.99 = 45.98
1 carbon atom
C
1 x 12.01 = 12.01
3 Oxygen atoms
O
3 x 16.00 = 48.00
105.99
Step 2: Find the percent of each element. Divide the part of the formula mass
that pertains to that element with the total formula mass
45.98
Percent of Potassium
K=
=
43.38 %
105.99 X 100
Percent of Carbon
C=
Percent of Oxygen
O=
12.01
X 100
105.99
48.00
X 100
105.99
Step 3: Check if the percentages add up to 100%
=
11.33 %
=
45.29 %
EMPIRICAL FORMULA
The empirical formula of a compound shows the simplest whole number ratio of
the atoms of each element present. It can be calculated from the masses of the
elements combined together and the relative atomic masses of its elements.
Example:
Suppose a compound is analyzed to contain 48.8 g of cadmium, 20.8 g of carbon, 2.62
g of hydrogen, and 27.8 g of oxygen. Determine the empirical formula of this
compound.
Step 1:
Step 2:
Step 3:
Convert the given mass to moles by dividing the mass of the element by
its atomic mass.
Get the simplest whole number ratio by dividing the number of moles of
each element by the smallest number of moles computed for in step1.
Use a multiplier if:
* After division if this step produces any number ending in .5 multiply all
numbers by 2 to obtain small whole numbers
* After division if this step produces any number ending in .33 multiply all
numbers by 3 to obtain small whole numbers
Use the whole number ratio as subscripts for the formula.
Mass
Atomic Mass
Moles (mass/molec.
mass)
Simplest whole number
ratio of moles
EMPIRICAL FORMULA
Cadmium, Cd
48.8 g
112.4g/mol
Carbon, C
20.8 g
12.01 g/mol
0.434 mol
1.67 mol
2.62 mol
1.74 mol
1.67 mol
2.62 mol
1.74 mol
(smallest no.
of moles)
0.434 mol
Hydrogen, H
2.62 g
1.01 g/mol
Oxygen, O
27.8 g
16.00 g/mol
0.434 mol
0.434 mol
0.434 mol
0.434 mol
=1
=4
=6
=4
CdC4H6O4
Empirical formulas can also be calculated from the percentage composition by
mass and the relative atomic masses of the elements. Assume that 100 g of the
compound is analyzed, having each percent as the mass in 100 g of the compound.
MOLECULAR FORMULA
The empirical formula gives you the simplest whole-number ratio of atoms, but
this does not tell you exactly the number of atoms of each element present in a
molecule of the compound. In order to know that information, we must solve for the
molecular formula. The molecular formula is an integer multiple of the empirical
formula. There are some compounds, like water (H20) that have the same empirical
formula and molecular formula, but generally most compounds do not.
For example:
The empirical formula of a white compound of phosphorus and oxygen was found to
be P2O5. Experimentation shows that the molar mass of this compound is 283.89g.
What is the compounds molecular formula?
Step 1:
Step 2:
formula.
Step 3:
Solve for the molecular mass of the Empirical formula.
Divide the given molecular mass by the molecular mass of the empirical
Use the quotient as a multiplier to the subscript of the empirical formula.
Empirical Formula = P2O5
Empirical Mass = 2(31.0) + 5(16.0) = 142g
Molecular Mass = 283.89g.
283.89
142
= 2
2(P2O5) = P4O10