How to Count Atoms

How to Count Atoms
1. The symbol of an element represents one atom of that element.
e.g., Ba =
2. A subscript is a number written at the lower right corner behind the symbol of an element. If
there is more than one atom of the element, then a subscript is used to indicate the number of
atoms.
e.g., Cl2 =
3. A subscript outside a bracket multiples all the elements inside the brackets.
e.g., Ca3(PO4)2
Ca = ________
P = ________
O = ________
3. A coefficient is a number written in front of a chemical symbol and indicates the number of
atoms of that element or number of molecules
e.g., 3C = ____
2NaCl = _______________________
2 H2O
_____ molecules of H2O
_____ H (hydrogen)
_____ O (oxygen)
3 Na2SO4
_____ molecules of Na2SO4
_____ Na (sodium)
_____ S (sulphur)
_____ O (oxygen)
4 Pb(NO3)2
_____ molecules of Pb(NO3)2
_____ Pb (Lead)
_____ N (nitrogen)
_____ O (oxygen)
Count the atoms present in the different compounds by using the coefficients and subscripts.
K2CO3
Type of Atom # of Atoms
______________ ______
______________ ______
______________ ______
Ba3(PO4)2
Type of Atom # of Atoms
_______________ ______
_______________ ______
_______________ ______
Pb(NO3)2
Type of Atoms # of Atoms
______________ ______
______________ ______
______________ ______
Na2CrO4
Type of Atom # of Atoms
______________ ______
______________ ______
______________ ______
3 CaCl2
Type of Atom # of Atoms
_______________ ______
_______________ ______
2 (NH4)2Cr2O7
Type of Atoms # of Atoms
______________ ______
______________ ______
______________ ______
______________ ______
NH4C2H3O2
Type of Atom # of Atoms
______________ ______
______________ ______
______________ ______
______________ ______
4 Al2(CO3)3
Type of Atom # of Atoms
________________ ______
________________ ______
________________ ______
Chemical Equations
We use equations to represent __________________________
2 Types:
1. __________________________:
potassium iodide + lead (II) nitrate
potassium nitrate + lead (II) iodide
2. _______________________:
-skeleton equation:
We often use subscripts in brackets to identify the __________ of a compound/element
(s)
(l)
(g)
(aq)
- _________________
- _________________
- _________________
- _________________________________
Law of Conservation of Mass
The law of conservation of mass states that the total mass of the reactants is always
__________________ to the total mass of the products
So... ____________________________________________________________
Because of this law, we must _________________ chemical equations. The molecules
are ________________ and the atoms are _________________but we must ensure
that there are equal numbers of atoms before and after the reaction.
i.e. if I start with 5 hydrogen atoms, I must end with ____________________________.
Rules for balancing:
1. Balance all ________________________ first
2. If possible, balance ____________________________ as a "chunk"
3. Balance any non-metals other than ____________________________
4. Balance _______________________
5. Balance _______________________
6. ___________________ both sides of the reaction to ensure all atoms are balanced!
Balance these reactions:
KI(aq) +
Pb(NO3)2(aq) 
KNO3(aq)
Al
H2SO4

Al(SO4)3
+
+
C3H8 +

FeCl3
CaO +
O2

CO2
Fe
+
Cl2
H2O

Ca(OH)2
H2O

Mg(OH)2 +
+
+
PbI2(s)
H2
H2O
Hint: Sometimes…
Mg
+
H2
Hint: If you have __________ on one side and __________ on the other... write H2O as
HOH and balance the _____________ ions first
C2H6 +
O2

CO2 +
H2O
Hint: If you end up with a half O2 molecule... _________________________
Classification of Chemical Reactions
Chemists have identified millions of different compounds, so there must be millions of different
chemical reactions to form them. When scientists are confronted with an overwhelming number
of things, they tend to classify them into groups, in order to make them easier to study and
understand. One popular classification scheme for chemical reactions breaks them up into five
major categories or types.
Types of Chemical Reactions:
1. Synthesis - A synthesis reaction involves two or more substances combining to make a more
complex substance. The reactants may be elements or compounds, and the product will always be
a compound. The general formula for this type of reaction can be shown as;
A
+
B
---->
AB
or
element or compound + element or compound -----> compound
Some examples of synthesis reactions are shown below;
2H2(g) + O2(g) ----> 2H2O(g)
C(s) + O2(g) ----> CO2(g)
CaO(s) + H2O(l) ----> Ca(OH)2(s)
2. Decomposition - In a decomposition reaction, one substance is broken down into two or more,
simpler substances. This type of reaction is the opposite of a synthesis reaction, as shown by the
general formula below;
AB
---->
A
+ B
or
Compound ------> element or compound + element or compound
Some examples of decomposition reactions are shown below;
C12H22O11(s) ----> 12C(s) + 11H2O(g)
Pb(OH)2(cr) ----> PbO(cr) + H2O(g)
2Ag2O(cr) ----> 4Ag(cr) + O2(g)
3. Single Displacement - In this type of reaction, a neutral element becomes an ion as it replaces
another ion in a compound. The general form of this equation can be written as;
In the case of a positive ion (cation) being replaced: A + BC ----> B + AC
or
In the case of a negative ion (anion) being replaced: A + BC ----> C + BA
in either case we have;
element + compound ----> element + compound
Some examples of single displacement reactions are shown below:
Zn(s) + H2SO4(aq) ----> ZnSO4(aq) + H2(g)
2Al(s) + 3CuCl2(aq) ---> 2AlCl3(aq) + 3Cu(s)
Cl2(g) + KBr(aq) ----> KCl(aq) + Br2(l)
4. Double Displacement - Like dancing couples, the compounds in this type of reaction exchange
partners. The basic form for this type of reaction is shown below;
AB + CD ----> CB + AD
or
Ionic Compound + Ionic Compound ----> Ionic Compound + Ionic Compound
Some examples of double displacement reactions are shown below;
AgNO3(aq) + NaCl(aq) ----> AgCl(s) + NaNO3(aq)
ZnBr2(aq) + 2AgNO3(aq) ----> Zn(NO3)2(aq) + 2AgBr(cr)
H2SO4(aq) + 2NaOH(aq) ----> Na2SO4(aq) + 2H2O(l)
5. Combustion - When organic compounds like propane are burned, they react with the oxygen in
the air to form carbon dioxide and water. The reason why these combustion reactions will stop
when all available oxygen is used up is because oxygen is one of the reactants. The basic form of
the combustion reaction is shown below;
hydrocarbon + oxygen ----> carbon dioxide and water
Some examples of combustion reactions are;
CH4(g) + 2O2(g) ----> 2H2O(g) + CO2(g)
2C2H6(g) + 7O2(g) ----> 6H20(g) + 4CO2(g)
C3H8(g) + 5O2(g) ----> 4H2O(g) + 3CO2(g)
Oxidation
When ______________________ reacts in a chemical reaction, the general name for
the reaction is oxidation.
The _________________ of the reaction will determine the name given to the reaction.
Corrosion: the ___________ reaction of oxygen with another substance
Metal and oxygen: forms a ________________________
Nonmetal and oxygen: forms a ___________________________________
Combustion: the _____________________ reaction of oxygen with a hydrocarbon
Complete combustion: the rapid reaction of oxygen with a hydrocarbon to form only
____________________________________________________ and energy. This
reaction happens when there is______________ of oxygen around and it consumes
_____________ of the fuel.
The flame is very hot and _________________.
Incomplete combustion: the rapid reaction of oxygen with a hydrocarbon to form water
and some combination of _______________________________________________
and energy. This reaction happens when there is _______________oxygen available.
The flame is cooler and ______________________.
Indicators
Acids and Bases
An acid is a substance that produces ___________________ in solution, __
(aq).
Examples:
Most solutions of acids or bases are ____ and _______. Therefore they cannot
be distinguished from ordinary water by appearance alone. The simplest way to
i) When hydrochloric acid, HCl is placed in solution it dissociates into
distinguish them from water is to use an indicator. An indicator is a substance
________________________
that changes _________ as the concentration of ___ and ___ changes.
ii) When sulfuric acid, H2SO4 is placed in water it dissociates into ________________________________
Indicator
Acids are created when _______________________ react with water.
SO2 + H2O 
NO2 + H2O 
A base is a substance that produces __________________ in solution, ___
Acid
Base
Neutral
Litmus Paper
Phenolpthalein
(aq).
Bromthymol Blue
Examples:
red cabbage
i) When sodium hydroxide, NaOH is placed in solution it dissociates into
__________________
Concentration of Acids and Bases/pH
ii) When calcium hydroxide, Ca(OH)2 is placed in solution it dissociates into
__________________
Concentration is defined as the amount of _______ per quantity of __________.
Bases are created when __________________________ react with water
Li2O + H2O 
FeO + H2O 
Characteristics of Acids & Bases
Substances have long been classified as acids or bases according to some general
properties which are summarized below.
Acid Properties
Base Properties
pH ________________________
pH ______________________
Conducts_________ when disolved in water
Conducts __________ when disolved in water
__________________ taste
________________ taste
corrosive
corrosive. slippery feel
The concentration of a product can easily be altered by
____________________ or the _______________________. ___________
is the universal solvent.
When you determine the concentration of hydrogen ions in solution (amount of H+
ions/ total solution volume) you are determining the ___ of that particular
solution. pH stands for, “the power of hydrogen”. The pH of a substance can be
determined a number of different ways, such as with the use of pH paper, an
electronic pH meter or mathematically. The pH scale ranges from _________.
0
7
Acids have a pH ______________
Bases have a pH ________________
Neutral substances have a pH _____________
14
The pH scale is actually a
measure of the number of
H+ ions in a solution. If
there are a lot of H+ ions,
the pH is very low. If
there are a lot of OHions, that means the
number of H+ ions is very
low, so the pH is high.
While the pH scale ranges from 0 to 14 and each pH unit represents a factor of
The pH of a substance depends on both _______________ (how many ions
dissociate?) and _____________________ (how much is dissolved in solution?)
Neutralization Reactions
Neutralization occurs when _________________ and ___________ are mixed
to make ______ and a ________. Neutralization reactions are a special type of
_____________ ________________ reaction. The general word equation for
a neutralization is:
_______.
A change in pH from 4 to 5 is a(n) _____________ increase/decrease in [H+]
A change in pH from 3 to 8 is a(n) _____________ increase/decrease in [H+]
A change in pH from 11 to 2 is a(n) _____________ increase/decrease in [H +]
Strength of Acids and Bases
Example:
Aqueous solutions of hydrobromic acid and beryllium hydroxide undergo a neutralization
reaction to produce water and beryllium bromide.
Strong acid –
Complete the following equations:
Example:
___ H2SO4 (aq) + ___ LiOH
(aq)

HCl(aq)  H+(aq) + Cl-(aq)
When hydrogen chloride molecules enter an aqueous solution, 100% of the
___ Ca(OH)2 (aq) + ___ H3PO4 (aq) 
hydrogen chloride molecules dissociate. As a result the solution contains the same
percent of H+ ions and Cl- ions: 100%
Strong base –
Which acid and base would you react together to produce the following salts:
i) KNO3
Examples: NaOH,
ii) MgCO3
Mg(OH)2