TEK 8(D)
:use the law of conservation of mass to write and balance chemical
equations;
The Law of conservation of mass states that
matter cannot be created or destroyed in any
chemical reaction
The atoms in the reactants are rearranged
to form new compounds, but none of the atoms
disappear, and no new atoms are formed.
H
H
H
H
Reactants
O
O
Products
A chemical equation
identifies the starting
and ending chemicals
as reactants and
products:
reactants products
Formation of water
2H2 + O2  2H20
A chemical equation is
balanced when it
reflects the
conservation of mass.
Graphic: http://www.chemistry.ohiostate.edu/betha/nealChemBal/
The same number of each kind of atom must
be on the left side of the arrow as are on the
right side when an equation is balanced.
Al
Al
Al
Al
O O
O O
O Al O Al O
O O
O Al O Al O
Balancing an equation:
2 H2
H
H
+
O2
O

O
2 H2O
H
O
H
Second:Inventory
First:
Determinethe
which
atoms
atoms
on both
are not
sides
equal
of the
and
add coefficients
yield arrow
to balance these atoms.
Hydogen
Hydogen
4H
2 atoms
H atoms
42
HH
atoms
atoms
atoms
atoms are
are =≠
2O
2 atoms
O atoms Oxygen
Oxygen
21
OO
atoms
atom
atoms are
are≠==
atoms
atoms
are
When
a coefficient
isinventory,
added
you
must
multiply
After
redoing
the
add
coefficients
Oxygen,
as
a
product,
must
be
multiplied
by 2.
This
is
now
a
balanced
equation
allto
in theother
compound
byWith
this each
number.
balanced
atoms.
additional
Aatoms
coefficient
is placed
in
front
of the
Redo
the inventory
coefficient
the inventory
must be updated.
compounds
that
contains O.
So: Chemical equations are balanced, when the
numbers and kinds of atoms on each side of
the reaction arrow are equal.
H
H
H
H
O
H
O
O
H
H
Reactants
Products
4 Hydrogen
4 Hydrogen
2 Oxygen
2 Oxygen
H
O
Because atoms are conserved, the mass of
the reactants in the chemical reaction is
equal to the mass of the products.
H2 + F2
H H
F
2g + 38g
40g

2 HF
H F
F
H F
= 2( 20g)
=
40g
Reactants
Products
40 g
40 g
The Law of Conservation of Mass allows
the calculation of the reactants or
products.
If 27.0 g of mercuric oxide (the red solid
compound) is heated and completely
decomposes to give the elements oxygen
and mercury. 2.0 g of oxygen are
produced. How many grams of mercury are
produced?
mercuric oxide  mercury + oxygen
HgO
 Hg
+
O2
2.0 g O2
27.0 g
HgO
? g Hg
27 g HgO = 2 g O2 + ? Hg = 25 g Hg
Reactants
Products
Which of the following chemical reactions
indicate that mass in conserved?
A. Mg + Cl2  2 MgCl
B. 2 Ca + O2  2 CaO
C. Zn + S  2 ZnS
D. C + O2  2 CO
Because atoms must be equal on each side
of the equation:
The correct answer is B
Ca
Ca
O
O
O
Ca
Ca
O
HCl
→→
MgCl
2 Mg + 2
HCl
MgCl
+ 2H2
2 +2 H
• When the above equation is balanced,
the coefficient for magnesium is —
• A 0
• B 1
The correct
• C 2
answer is C
• D 4
Suppose 100 g of iron metal rusts. We weigh
the rust and find that the rust has a mass of
143 g. What mass of oxygen reacted with the
iron?
A. 243 g
B. 57 g
C. 100 g
D 43 g
Iron + Oxygen  Rust
• 100 g + ?g  143g
• What mass of
oxygen is needed? 43 g Oxygen Choice D