Counting Atoms and Balancing Equations

Balancing Chemical

Equations

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Counting Atoms

Balancing Equations

Law of Conservation of

Mass

OBJECTIVES:

This lecture focusses on
the application of the
Law of Conservation of
M a s s i n b a l a n c i n g
chemical equations.

• To discuss the Law of

Conservation of Mass

• To count the atoms

and balance chemical
equations

Counting Atoms

1) 2CrCl32) Ba(OH)23) 3Ca(ClO3)2

Counting Atoms - Multiply coefficient by subscript
of each element.
• If in parenthesis, the subscript after the
parenthesis is multiplied to the chunk of
everything in the parenthesis than multiplied by
coefficient.

Cr – (2x1) = 2
Cl – (2 x 3) = 6

Ba – 1
O – (2x1) = 2
H – (2 x 1) = 2

Ca – (3x1) = 3
Cl – (2 x 3) = 6
O – (3 x 3 x 2) = 18

2CrCl₃

Cr=? and Cl=?

2CrCl₃

Cr=2 and Cl=6

Fill in the Blanks

Ba(OH)₂

Ba=_

O=_

H=_

Type your answer separating the first, second and third with a comma.

Example: 1, 2, 3

Fill in the Blanks

3Ca(ClO₃)₂

Ca=_

Cl=_

O=_

Type your answer separating the first, second and third with a comma.

Example: 1, 2, 3

Counting Atoms

Counting Atoms - Multiply coefficient by subscript
of each element.
• If in parenthesis, the subscript after the
parenthesis is multiplied to the chunk of
everything in the parenthesis than multiplied by
coefficient.

Cr – (2x1) = 2
Cl – (2 x 3) = 6

Ba – 1
O – (2x1) = 2
H – (2 x 1) = 2

Ca – (3x1) = 3
Cl – (2 x 3) = 6
O – (3 x 3 x 2) = 18

1) 2CrCl32) Ba(OH)23) 3Ca(ClO3)2

Balancing chemical equations

1. Every atom in _________becomes part of the

___________.

2. Atoms are never ___________ or _________
3. Scientists know that there must be the ________

number and type of atoms on each side of the
equation.

3. Balance by using ____________ in front of the

chemical formulas because you can not add or change
___________of a correct formula

reactants

products

created

destroyed

coefficients

subscripts

same

Steps to Balancing Equations

1) Determine the number of atoms for each element.
2) Pick an element that is not equal on both sides of the equation.
3) Add a coefficient in front of the formula with that element and

adjust your counts.

4) Continue adding coefficients to get the same number of atoms of

each element on both sides

Example

Step 1: Determine the number of atoms for each element.

Mg + O2 MgO

Mg = 1 Mg = 1
O = 2 O = 1

Example

Step 2: Pick an element that is not equal on both sides
of the equation.

Mg + O2 MgO

Mg = 1 Mg = 1
O = 2 O = 1

Since the O atoms
are not equal, we’ll
target those first.

LAW OF CONSERVATION OF MASS

Law of Conservation of mass – MASS CANNOT BE CREATED OR
DESTROYED DURING A CHEMICAL REACTION.
•

The mass of all the REACTANTS combined must equal the mass of all

the PRODUCTS combined
1. Antoine Lavoisier came up with this law

How does a balanced equation
illustrate the Law of Conservation of
Mass

• A balanced chemical equation shows the same

number and kind of atoms in both the reactants and
the products.

• Therefore, the total mass of all the atoms in the

reactants is the same as the total mass of the same
atoms in the products.

Let’s practice balancing

equations!

REMEMBER:
A balanced chemical equation shows the same
number and kind of atoms in both the reactants and
the products.

Balance the following chemical equations:

Fill in the Blanks

_C₃H₈ + _O₂ -> _CO₂ + _H₂O

Example:

1, 2, 3, 4

Fill in the Blanks

_Na + _H₂O -> _NaOH + _H₂

Example:

1, 2, 3, 4

Fill in the Blanks

_C₂H₆ + _O₂ -> _CO₂ + _H₂O

Example:

1, 2, 3, 4

Fill in the Blanks

_Ca + _AlCl₃ -> _CaCl₂ + _Al

Example:

1, 2, 3, 4

Fill in the Blanks

_Al + _Cu(NO₃)₂ -> _Al(NO₃)₃ + _Cu

Example:

1, 2, 3, 4

Creating balanced equations from sentences

Fill in the Blanks

Solid iron and chlorine gas form iron (III) chloride.

Type the formula for the reactants.

Example:

Na+H2O (no spacing, consider the chemical symbol and the subscript)

Fill in the Blanks

Solid iron and chlorine gas form iron (III) chloride.

Type the formula for the product.

Example:

H2O (no spacing, consider the chemical symbol and the subscript)

Fill in the Blanks

_Fe + _Cl₂ -> _FeCl₃

Example:

1, 2, 3