The document discusses the law of conservation of mass and balancing chemical equations. It explains that in a chemical reaction, while the types of atoms may change, the total mass remains the same. It then provides examples of balancing different chemical equations by making the numbers of atoms equal on both sides of the reaction arrow. Methods for balancing include multiplying coefficients and accounting for polyatomic ions.

1. LAW OF CONSERVATION OF
MASS

2. LAW OF CONSERVATION OF MASS
?
Given the following chemical reaction:
CaCl2 + Na2SO4  CaSO4 + NaCl
reactants = 300.23 g products = ?
What is the mass of the products?
300.23 g

3. LAW OF CONSERVATION OF MASS
Law of Conservation of Mass:
Matter cannot be created or destroyed.

4. LAW OF CONSERVATION OF MASS
In a chemical reaction, although matter may change
in form, its mass does not change.

5. LAW OF CONSERVATION OF MASS
Thus when writing a chemical equation, all atoms
must be balanced between the reactants and the
products.

6. LAW OF CONSERVATION OF MASS
Ways of writing chemical reactions:
1. Word equation:
Sodium + chlorine gas  sodium chloride
2. Sentence:
Sodium and chlorine gas yield sodium chloride
3. Chemical/skeleton equation:
2Na(s) + Cl2(g)  2NaCl(s)

7. LAW OF CONSERVATION OF MASS
The arrow symbol:
2Na(s) + Cl2(g) 2NaCl(s)
- Means “yields”
- CANNOT be replaced with an equal sign (=)

8. BALANCING CHEMICAL
EQUATIONS

9. Balancing Chemical
Equations
H2O2  H2O + O2
H
O
2
2
H
O
2
3

10. Balancing Chemical
Equations
H2O2  2H2O + O2
H
O
2
2
H
O
4
4

11. Balancing Chemical
Equations
2H2O2  2H2O + O2
H
O
4
4
H
O
4
4
BALANCED!!!

12. Balancing Chemical
Equations
RE-DO:
H2O2  H2O + O2
H
O
2
2
H
O
2
3
You may be tempted to put a 3 in
front of the H2O2…let’s try that!

13. Balancing Chemical
Equations
RE-DO:
3H2O2  H2O + O2
H
O
6
6
H
O
2
3

14. Balancing Chemical
Equations
RE-DO:
3H2O2  3H2O + O2
H
O
6
6
H
O
6
5

15. Balancing Chemical
Equations
RE-DO:
15H2O2  18H2O + O2
H
O
30
30
H
O
32
20
GETS HARDER AND HARDER!!!!!!!
If it doesn’t seem to work, start again using DIFFERENT
coefficients

16. Balancing Chemical
Equations
H2O2  H2O + O2
H
O
2
2
H
O
2
3
When you see an odd number, just
multiply the compound that is making
the number odd by 2 to make it even,
and easier to work with
2
X 4
X 4

17. Balancing Chemical
Equations
CaSO4 + KOH  Ca(OH)2 + K2SO4
Ca
SO4
K
OH
1
1
1
1
Ca
SO4
K
OH
1
1
2
2
Polyatomic ions!!!!
You can think of polyatomic ions as INDIVIDUAL
elements when completing the chart

18. Balancing Chemical
Equations
CaSO4 + 2KOH  Ca(OH)2 + K2SO4
Ca
SO4
K
OH
1
1
2
2
Ca
SO4
K
OH
1
1
2
2

19. Balancing Chemical
Equations
CaSO4 + 2KOH  Ca(OH)2 + K2SO4
Ca
SO4
K
OH
1
1
2
2
Ca
SO4
K
OH
1
1
2
2
BALANCED!!!

20. Balancing Chemical
Equations
H3PO4 + NaOH  H2O + Na3PO4
You can pretend that each polyatomic ion is an
INDIVIDUAL element when making the chart

21. Balancing Chemical
Equations
H3PO4 + NaOH  H2O + Na3PO4
H
PO4
Na
O
4
1
1
1
H
PO4
Na
O
2
1
3
1

22. Balancing Chemical
Equations
H3PO4 + 3NaOH  H2O + Na3PO4
H
PO4
Na
O
6
1
3
3
H
PO4
Na
O
2
1
3
1

23. Balancing Chemical
Equations
H3PO4 + 3NaOH  3H2O + Na3PO4
H
PO4
Na
O
6
1
3
3
H
PO4
Na
O
6
1
3
3
BALANCED!!!

24. Balancing Chemical
Equations
BaCl2 + (NH4)2CO3  BaCO3 + NH4Cl
You can substitute each polyatomic ion with an
X, Y, or Z to make life a little easier…
BaCl2 + (NH4)2CO3  BaCO3 + NH4Cl
X Y

25. Balancing Chemical
Equations
BaCl2 + (X)2Y  BaY + XCl
Ba
Cl
X
Y
1
2
2
1
Ba
Cl
X
Y
1
1
1
1

26. Balancing Chemical
Equations
BaCl2 + X2Y  BaY + 2XCl
Ba
Cl
X
Y
1
2
2
1
Ba
Cl
X
Y
1
2
2
1
BALANCED!!!
...but not done yet

27. Balancing Chemical
Equations
Ba
Cl
NH4
CO3
1
2
2
1
Ba
Cl
NH4
CO3
1
2
2
1
BaCl2 + (NH4)2CO3  BaCO3 + 2NH4Cl
DON’T KEEP X’s and Y’s in your answer.
Replace them with the polyatomic ions.
CORRECT!

28. Balancing Chemical
Equations
zinc sulfide + oxygen gas  zinc oxide + sulfur dioxide
WORD EQUATION
Convert this to a chemical equation (also known as a
“skeleton equation”).
This is when you use your naming/formula-writing rules!!!
ZnS + O2  ZnO + SO2
CHEMICAL EQUATION

29. Balancing Chemical
Equations
ZnS + O2  ZnO + SO2
Zn
S
O
1
1
2
Zn
S
O
1
1
3
Make this number
even by multiplying
the compound that’s
causing the oddness
by 2

30. Balancing Chemical
Equations
ZnS + O2  2ZnO + SO2
Zn
S
O
1
1
2
Zn
S
O
2
1
4

31. Balancing Chemical
Equations
2ZnS + O2  2ZnO + SO2
Zn
S
O
2
2
2
Zn
S
O
2
1
4

32. Balancing Chemical
Equations
2ZnS + O2  2ZnO + 2SO2
Zn
S
O
2
2
2
Zn
S
O
2
2
6

33. Balancing Chemical
Equations
2ZnS + 3O2  2ZnO + 2SO2
Zn
S
O
2
2
6
Zn
S
O
2
2
6
BALANCED!!!