Percent Composition and
Chemical Formulas
STEM_GC11PC-If-31

Percentage of the class planning:
1.Get total number of learners in class.
2. Get the number of learners who would like
to pursue engineering in college. Get the
percentage.
3. Get the number of learners who would like
to pursue the sciences. Get the percentage.
4. Get the number of learners who would like
to pursue mathematics. Get the percentage

What information can be obtained
from a chemical formula? For
example, what information can you
get from the formula of carbon
dioxide, CO2?

a. The compound is made up of two
elements, namely carbon and oxygen.
b. One molecule of CO2 is made up of
one atom of carbon and two atoms of
oxygen.
c. One mole of CO2 molecules will have
one mole of C atoms and two moles of O
atoms.

c. The ratio of the moles of C to the
moles of O in CO2 is 1:2.
d. CO2 is composed of 27.29% carbon
and 72.71% oxygen. The chemical
formula provides the percent composition
of CO2.

The percent composition by mass
is the percent by mass of each
element in a compound.
Mathematically,

Solve for CO2

Answer the following practice exercises:
I. Calculate the percent composition of
NaCl.
II. The chemical formula of glucose is
C6H12O6. Determine its percent
composition.
III. Which element comprising Mg(OH)2
has the highest percentage by mass?

Answer Key
1. 39.34% Na, 60.66% Cl
2. 39.99% C, 6.727% H,
53.28% O
3. O; the composition is
41.68% Mg, 54.89% O, and
3.46% H

Empirical Formula from
Percent Composition

The empirical formula of a
compound can be calculated
from the percent composition.
Because percentage is given, it
is convenient to assume 100.00
grams of the compound.

Illustrate using the following examples:
A compound is found to consist of 7.81%
C and 92.19% Cl. What is the empirical
formula of the compound?

Assume 100.00 grams of the
compound. The sample will therefore
contain 7.81 g C and 92.19 g Cl. The
grams are converted to moles to get
the ratios of the moles of the
elements in the compound:

The compound is C0.650 Cl2.601. But
chemical formulas are expressed
in whole numbers.

Empirical formulas are expressed as the
lowest whole number ratio between the
atoms.
To convert to whole numbers, divide the
number of moles by the smallest value (that
is
0.650).

A compound is found to consist of
43.64% P and 56.36% O. The molar
mass for the compound is 283.88 g/mol.
What is the empirical formula and
molecular formula of the compound?
Assume 100.00 grams of the compound.
What is the mass of each element in
100.00 grams of compound?

What are the moles of each element in
100.00 grams of compound?

The compound is PO2.5. But the
subscripts are still not whole
numbers. Multiply the subscripts by a
factor to get the smallest whole
number. When multiplied by 2, the
empirical formula is P2O5

What is the molecular formula?
Compare the mass of the empirical
formula to the molar mass:

Aspirin has the molecular formula C9H8O4.
_____1. What is the % C in aspirin by mass in
aspirin?
_____2. What is the % O in aspirin by mass in
aspirin?
_____3. An oxide of chromium is made up of 5.20 g
chromium and 5.60 g oxygen What is the
empirical formula of the oxide? (Note: An oxide of
nitrogen contains 63.1% oxygen and has a
molar mass of 76.0 g/mol.)
_____4. What is the empirical formula for this
compound?
_____5. What is the molecular formula of the
compound?

1. 60.00%
2. 35.53%
3. Cr2O7
4. N2O3
5. N2O3

Chemical Reactions
and
Chemical Equations
(Lecture)

What does the following have in
common:
a. Rusty iron nail
b. Change in color of leaves
c. Bleached hair

Writing and Balancing
a Chemical Equation

In a chemical reaction, a
substance (or substances) is
converted to one or more new
substances. Chemical reactions
follow the law of conservation of
mass. No atoms are created or
destroyed; they are just
rearranged.

Chemists have a way of
communicating chemical
reactions. They represent
chemical reactions through
chemical equations.

Consider the reaction of hydrogen
gas (H2) with chlorine gas (Cl2) to
yield hydrogen chloride.

We can represent this reaction through a chemical
equation. The reactants (starting substances)
are placed on the left side. The products
(substances produced) are placed on the right.

An arrow points towards the direction of the
reaction. The equation has to be
balanced so that the same number and types of
atoms appear on the left and right side of the
equation

To balance, coefficients
(numbers preceding the
chemical formula) are
used.

Hence, the balanced chemical equation is:

When a substance is placed in water, we
indicate this with aq, meaning it is in an
aqueous environment. For example, when
KBr reacts with AgNO3 in an aqueous
environment, KNO3
and solid AgBr are produced. This reaction is
represented as:

Sample Problem:
Ethane (C2H6) reacts with
oxygen gas (O2) to produce
carbon dioxide and water.
Write the balanced chemical
equation for the reaction.

Identify reactants and products and
write their correct formulas. Put
reactants on the left side and
products on the right.

Balance the equation by changing the
coefficients of the reactants or products. Do
not change the subscripts or the chemical
formula.
To use the smallest whole number
coefficients, we multiply the equation by 2 to
give:

Check to make sure that the number of
each type of atom is the same on each
side of the equation.

HOW TO BALANCE
EQUATIONS IN A NUTSHELL:
1. Balance the metals
2. Balance the nonmetals
except H & O
3. Balance the Hydrogen then
the Oxygen.

SEATWORK:
Balance the following equations .

Answer Key
1. 2, 1, 2
2. 2, 1, 2
3. 2, 2, 1
4. 1, 2, 1, 2
5. 2, 2, 1

Types and Evidences that a Chemical
Reaction has Occurred
Here are some evidences that a chemical
reaction has occurred:
a. Change in color
b. Formation of a solid (a precipitate)
c. Evolution of gas (bubble formation)
d. Change in temperature (heat is
released or absorbed)

Most chemical
reactions can be
classified into five
types:

Types and Evidences
that a Chemical
Reaction has Occurred

Here are some evidences that a chemical reaction has
occurred:
a. Change in color
b. Formation of a solid (a precipitate)
c. Evolution of gas (bubble formation)
d. Change in temperature (heat is released or absorbed)

What does the following have in
common:
a. Rusty iron nail
b. Change in color of leaves
c. Bleached hair

Most chemical reactions can be classified
into five types:
1. Decomposition reaction – a reactant
breaks down into two or more products

2. Synthesis reaction – two or more
reactants form a single product

3. Single displacement reaction – one
element replaces another in a
compound

4. Double displacement – two ionic
compounds exchange ions

5. Combustion reaction – a
hydrocarbon (a compound containing
carbon and hydrogen) reacts with oxygen
to form carbon dioxide and water.