The document explains how to symbolize molecules, indicating the number of atoms in compounds and providing examples of molecular formulas. It discusses the law of conservation of mass, emphasizing that atoms are rearranged in chemical reactions and must be balanced on both sides of a chemical equation. The document includes various exercises for balancing chemical equations using a method of inspection.

2. How molecules are symbolized
Cl2 2Cl 2Cl2
• Molecules may also have brackets to
indicate numbers of atoms. E.g. Ca(OH)2
O
H
O
H
Ca
• Notice that the OH is a group
• The 2 refers to both H and O
• How many of each atom are in the following?
a) NaOH
b) Ca(OH)2
c) 3Ca(OH)2
Na = 1, O = 1, H = 1
Ca = 1, O = 2, H = 2
Ca = 3, O = 6, H = 6

3. Balancing equations: MgO
• The law of conservation of mass states that
matter can neither be created or destroyed
• Thus, atoms are neither created or destroyed,
only rearranged in a chemical reaction
• Thus, the number of a particular atom is the
same on both sides of a chemical equation
• Example: Magnesium + Oxygen (from lab)
• Mg + O2  MgO O
Mg O
+  Mg O
• However, this is not balanced
• Left: Mg = 1, O = 2
• Right: Mg = 1, O = 1

4. Balance equations by “inspection”
Hints: start with elements that occur in one
compound on each side. Treat polyatomic ions
that repeat as if they were a single entity.
5
2 3
3.5
2 7 4 6
2 2
2
2 6
3
C2H6 + O2  CO2 + H2O
a) P4 + O2  P4O10
b) Li + H2O  H2 + LiOH
c) Bi(NO3)3 + K2S  Bi2S3 + KNO3
d) C2H6 + O2  CO2 + H2O
From Mg + O2  MgO
2Mg + O2  2MgO is correct
Mg + ½O2  MgO is
incorrect
Mg2+ O2  2MgO is incorrect
4Mg + 2O2  4MgO is incorrect

5. a) Mg + 2HCl  MgCl2 + H2
b) 3Ca + N2  Ca3N2
c) NH4NO3  N2O + 2H2O
d) 2BiCl3 + 3H2S  Bi2S3 + 6HCl
e) 2C4H10 + 13O2  8CO2 + 10H2O
f) 6O2 + C6H12O6  6CO2 + 6H2O
g) 3NO2 + H2O  2HNO3 + NO
h) Cr2(SO4)3+ 6NaOH  2Cr(OH)3+ 3Na2SO4
i) Al4C3 + 12H2O  3CH4 + 4Al(OH)3
Balance these skeleton equations:

6. a) 2KNO3  2KNO2 + O2
b) 2Pb(NO3)2  2PbO + 4NO2 + O2
c) P4 + 6I2  4PI3
d) 3MgO + 2H3PO4  Mg3(PO4)2 + 3H2O
e) Br2 + 2KI  I2 + 2KBr
f) Ca(OH)2 + 2HNO3  Ca(NO3)2 + 2H2O
g) Bi2O3 + 3H2  2Bi + 3H2O
h) 3Fe + 2O2  Fe3O4
i) 2CaO + 5C  2CaC2 + CO2
Here are some more to balance:

7. Question 3 pg. 252
a) 2Li + 2H2O  H2 + 2LiOH
b) P4 + 5O2  P4O10
c) 2C2H6 + 7O2  4CO2 + 6H2O
d) CS2 + 3O2  CO2 + 2SO2
e) 2AsCl3 + 3H2S  As2S3 + 6HCl
f) 3AgNO3 + FeCl3  3AgCl + Fe(NO3)3
g) 2KClO3  2KCl + 3O2
h) 2SO2 + O2  2SO3
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