Counting Atoms
Balancing Equations
Law of Conservation of Mass
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.
1) 2CrCl3 2) Ba(OH)2 3) 3Ca(ClO3)2
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
Balancing chemical equations
reactants
1. Every atom in _________becomes
part of the
products
___________.
created
destroyed
2. Atoms are never ___________
or _________
3. Scientists know that there must be the
same number and type of atoms on each
________
side
________
of the equation.
3. Balance by using ____________
coefficients in front of
the chemical formulas because you can not
subscripts
add or change ___________of
a correct
formula
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
O=2
Mg = 1
O= 1
Example
Step 2: Pick an element that is not equal on both
sides of the equation.
Mg + O2
MgO
Mg = 1
O=2
Mg = 1
O= 1
Since the O atoms
are not equal, we’ll
target those first.
Example
Step 3: Add a coefficient in front of the formula
with the element and adjust your counts.
Mg + O2
Mg = 1
O=2
2 MgO
Mg = 1 2
O= 12
Adding a 2 in front
of MgO will change
the number of
atoms on the
product side of the
equation.
Example
Step 4: Continue adding coefficients to get the
same number of atoms of each element on each
side.
2 Mg + O2
2 MgO
Mg = 1 2
O=2
Mg = 1 2
O= 12
Now we need to increase the number of Mg atoms we have on
the reactants side. Adding a coefficient 2 in front of Mg will give
us 2 atoms of Mg and balance the equation.
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
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.
It’s Your turn
• Balance the following equations: