Formulas and Percent
Composition
Finding the Mystery Formulas
Using Analytical Data
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Scientist synthesize new compounds for many
uses.
Once they make a new product, they must check
its identity.
One way to carry out a chemical analysis is by
providing a percent composition.
Example in History
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In 1962, scientists made a new compound from xenon
and fluorine.
Before 1962, scientist believed xenon did not form
compounds.
The scientists did a percent composition of the
compound and found it was 63.3% Xe and 36.7 % F,
which is the same as that for a formula XeF4.
Percent Composition helps identify a substance by
comparing ratio of masses (empirical formula).
Determining % Composition
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% Composition =
Mass of part of sample
Mass of whole sample
X 100
*% composition remains consistent regardless of the size of the
sample
Example % Comp Problem
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Ex. Find the % composition of Ca, O and H
in a Ca(OH)2 sample.
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Based on a 20.18 g sample of Ca(OH)2, how
many grams did the individual elements
contribute?
Determining Empirical Formulas
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Data for a percent composition allow you to
calculate the simplest ratio among the atoms
found in a compound.
Empirical Formula is the simplest ratio of
atoms in a compound
Ammonium Nitrite formula is NH4NO2
Ammonium Nitrite empirical Formula is NH2O
Percent Composition To Empirical
Formula
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*Step 1 – change % to grams, assuming a 100 g sample
*Step 2 – convert grams to moles
*Step 3 – divide all mole values by the smallest mole
value to try and get all values into simple whole
numbers
*Step 4 – if mole values are not whole numbers after
step 3, multiply all mole values by the same smallest
whole number to achieve this
*Step 5 – write the empirical formula of the compound
Example Problems
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A compound is found to have 79.8% carbon
and 20.2% hydrogen by mass. What is the
empirical formula of this compound?
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A compound is composed of 58.8% C, and
9.8% H. What is the empirical formula of this
compound?
More Practice Problems
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60.0% C, 13.4% H, 26.6% O. Find the
empirical formula.
A dead battery is found to contain a compound
of 69.9% Mn and 30.4% O. What is the
empirical formula?
Magnetic iron oxide is 72.4% iron and 27.6%
oxygen. What is the empirical formula?
Empirical formulas not molecular
formulas
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In determining empirical formulas, the simplest
ratio of atoms was determined. This is not
necessarily the exact number of atoms in the
compound which is known as the molecular
formula.
Additional information is needed to determine
molecular formulas.
Molecular Formulas
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Molecular Formulas are made of single
molecules.
For many compound the molecular formula is
a simple whole number ratio of the empirical
formula
The molar mass of a molecular formula is equal
to the molar mass of a empirical formula
multiplied by “n” (a whole number)
Process
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If you divide the experimental molar mass by
the molar mass of the empirical formula, you
can figure out the value of n needed to scale the
empirical formula up to give the molecular
formula.
n(emperical formula) = molecular formula
Example Problem
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The empirical formula of a compound is CH3N.
The mass of the molecular formula is 60.12
g/mol. What is the molecular formula of this
compound?
1.
2.
3.
Steps To Find The Molecular
Formula From The Empirical
Formula
Find molar mass of the empirical formula
Divide the molecular mass given in the problem by
the empirical formula mass to get the multiple of how
many times larger the molecular mass is compared to
the empirical mass (n).
Use the multiple you found in step 2 to write your
molecular formula. This must mean that the # of
atoms of the empirical formula must be multiplied by
this multiple
n(empirical formula)
More Example Problems
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Molar mass of 78 g/mol and empirical formula
of CH, what is its molecular formula?
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A brown gas on the cabinet to your right has the
empirical formula of NO2. Its experimental
molar mass is 46g/mol. What is its molecular
formula?
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The lichens presents on the bark of trees contain
a substance called succinic acid. The organic acid
is also present in fungi, which is used to make
dyes and pefumes.
Percent composition is 40.68% C, 5.08% H,
54.24 % O.
Its molecular mass is 118.8 g/mol
What is its empirical and molecular formulas?