Chapter 3

• Chefs have recipes, chemists have recipes.
• Recipes in chemistry can be seen on chemical
equation.
• Instead of using cups and teaspoons, chemists use
moles.
• Instead of eggs, butter, sugar, etc. Chemists use
chemical compounds as ingredients.

How to make
chocolate chips cookies?
Ingredients:
- a cup of butter
- a half cup of sugar
- a cup of brown sugar
- a teaspoon of vanilla
- 2 pieces of eggs
- 2,5 cups of flour
- a teaspoon of baking soda
- a teaspoon of salt
- 2 cups of chocolate chips
How to make a soap?
Ingredients:
- 75 grams of texapon
- 30 grams of coconut oil
- 30 grams of glycerin
- 50% of Salt solution
- 50% of Citric acid
- and so on….

• Reaction equation tells us about what you need to react (reactant) to get a product. (like the cookies recipe)
• STOICHIOMETRY is derived from Greek languages: stoicheion (element) and metron
(measure)
• Usage: STOICHIOMETRY is used to measure the amount of substances involved in chemical reactions.

CH
4
+ 2O
2
 CO
2
+ 2H
2
0
• This reaction tells us that by mixing 1 mole of methane with 2 moles of oxygen we will get 1 mole of carbon dioxide and 2 moles of water.
• If we want to get 10 moles of water, how many moles of methane and oxygen is needed? How many grams of CO
2 is produced?

• The unit of measurement which is used to count the number of atoms, molecules, or particles.
• 1 mole of any substance = N
A atoms, molecules, or particles.
= 6.02 x 10 23
• e.g. 1 mole of silver = 6.02 x 10 23 atoms of silver

Particles in a Mole
Amedeo Avogadro (1766-1856)
(1776 – 1856) it was named in his honor by a
French scientist in 1909. its value was first estimated by Josef Loschmidt, an Austrian chemistry teacher, in 1895
.
?
quadrillions trillions billions millions thousands
1 mole = 602213673600000000000000 or 6.022 x 10 23
There is Avogadro's number of particles in a mole of any substance.

Mole
1 mole of Na = 6.02 x 10 23 atoms of
Na
1 mole water = 6.02 x 10 23 molecules of water other
1 dozen of eggs = 12 pieces of eggs
A pair of shoes = 2 pieces of shoes
1 mole of HCl = 22,4 L HCl (STP) 1 rims of paper = 500 sheets of paper

• When we measure one mole of a substance on a balance, it called “molar mass” and the unit is g/mol (gram per mole).

• carbon 12.01 g/mol
• aluminum 26.98 g/mol
• zinc 65.39 g/mol

• water

H
2
O

2(1.01) + 16.00 = 18.02 g/mol
• sodium chloride

NaCl

22.99 + 35.45 = 58.44 g/mol

• sodium bicarbonate

NaHCO
3

22.99 + 1.01 + 12.01 + 3(16.00)
• sucrose = 84.01 g/mol

C
12
H
22
O
11

12(12.01) + 22(1.01) + 11(16.00)
= 342.34 g/mol

Particles
٪ N
A x N
A
Mole
X
MM
٪ MM
MM = Molar Mass
NA = 6.02 x 10 23
STP = Standard
Temperature
Pressure
22,4L ٪ X
22,4L
Volume
(STP)
Mass

1.
Write and balance the equation
2.
Write down all given information
3.
Convert everything into moles
4.
Use mole ratio to solve the problem
5.
Convert everything into the required unit
(Mass, particles, volume)

• Mole ratio is based on the coefficient of the balanced chemical equation.
• e.g. CH
4
+ 2O
2
 CO
2
+ 2H
2
O
• Remember :
“The ratio of coefficient = the ratio of mole”
• The mole ratio = 1 : 2 : 1 : 2

H
2
+ O
2
 H
2
O (not balance)
Question:
• If 3 moles of oxygen are completely react with hydrogen, how many grams off water produced?
• If 72 grams of water are produced, how many moles of oxygen are needed?

N
2
+ H
2
 NH
3
(not balance)
Question:
• How many molecules of ammonia are produced when 2 grams of nitrogen is reacted with hydrogen ?
• How many grams of oxygen are needed to produce 10 grams of ammonia?

Ethane gas (C
2 reaction below:
H
6
) is burnt at STP by following a
C
2
H
6
+ O
2
 CO
2
+ H
2
O (not balance)
Question:
• How many liters of oxygen are needed to burn 12 moles of C
2
H
6
?
• How many liters of CO
2
14 moles of O
2
?
are produced if we burn

Camels store the fat tristearin (C
57
H
110
O
6
) in the hump. As well as being a source of energy, the fat is a source of water, because when it is used the reaction
2 C
57
H
110
O
6
(s) + 163 O
2
(g)  114 CO
2
(g) + 110 H
2
O(l) takes place. What mass of water can be made from 1.0 kg of fat?

The compound diborane (B
2
H
6
) was at one time considered for use as a rocket fuel. How many grams of liquid oxygen would a rocket have to carry to burn
10 kg of diborane completely?
(The products are B
2
O
3 and H
2
O).
Chemical equation B
2
H
6
+ O
2
Balanced chemical equation B
2
H
6
+ O
2
10 kg x g
B
B
2
2
O
3
O
3
+ H
2
O
+ H
2
O

Modified by Apollo 13 Mission
Astronaut John L. Swigert holds the jury-rigged lithium hydroxide scrubber used to remove excess carbon dioxide from the damaged Apollo 13 spacecraft.

Formula: n = number of element
Percentage composition tell you the percent of
mass of the element which made up the compound.

Calculate the percentage composition of C and N in urea,
CO(NH
2
)
2
.
Answer:
Molar mass of urea = Ar C + Ar O + (2 x Ar N) + (4 x Ar H)
= 12 + 16 + 28 + 4
= 60
% C = (1 x 12) x 100% = 20%
60
% N = (2 x 14) x 100% = 46.67%
60

1. Calculate the percentage composition of nitrogen in :
(NH
NH
4
4
)
2
SO
4
NO
3
2. Determine the mass of nitrogen in:
- 100 grams of Ca(NO
3
- 200 grams of (C
2
H
5
)
2
)
2
NH

3. Calculate the percentage composition of carbon and oxygen in :
C
6
H
12
O
6
CH
3
OCH
3
4. Chlorophyll contains 4.8% of magnesium. Assume that each molecules of chlorophyll contain 1 atom Mg. determine the relative molecular mass (Mr) of chlorophyll. (Ar Mg=24)

• Available Ingredients
▫ 4 slices of bread
▫ 1 jar of peanut butter
▫ 1/2 jar of jelly
• Limiting Reactant
– bread
• Excess Reactants
– peanut butter and jelly

• Limiting Reactant
▫ used up in a reaction
▫ determines the amount of product
• Excess Reactant
▫ added to ensure that the other reactant is completely used up
▫ cheaper & easier to recycle

Limiting Reagents
6 green used up
6 red left over
3.9

• Pick A Product
• Try ALL the reactants
• The lowest answer will be the correct answer
• The reactant that gives the lowest answer will be the limiting reactant

• 10.0g of aluminum reacts with 35.0 grams of chlorine gas to produce aluminum chloride. Which reactant is limiting, which is in excess, and how much product is produced?
2 Al + 3 Cl
2
• Start with Al:
 2 AlCl
3
10.0 g Al 1 mol Al 2 mol AlCl
3
133.5 g AlCl
3
27.0 g Al 2 mol Al 1 mol AlCl
3
• Now Cl
2
:
= 49.4g AlCl
3
35.0g Cl
2
1 mol Cl
2
2 mol AlCl
3
133.5 g AlCl
3
71.0 g Cl
2
3 mol Cl
2
1 mol AlCl
3
= 43.9g AlCl
3

• Convert one of the reactants to the other
REACTANT
• See if there is enough reactant “A” to use up the other reactants
• If there is less than the GIVEN amount, it is the limiting reactant
• Then, you can find the desired species

Do You Understand Limiting Reagents?
g Al g Fe
2
O
3
In one process, 124 g of Al are reacted with 601 g of Fe
2
O
3
2Al + Fe
2
O
3
Calculate the mass of Al
2
O
3 formed.
Al
2
O
3
+ 2Fe mol Al mol Fe
2
O
3 mol Fe
2
O
3 needed
OR mol Al needed g Fe
2
O
3 needed g Al needed
124 g Al x
1 mol Al
27.0 g Al x
1 mol Fe
2
O
3
2 mol Al x
160. g Fe
2
O
3
1 mol Fe
2
O
3
= 367 g Fe
2
O
3
Start with 124 g Al need 367 g Fe
2
O
3
Have more Fe
2
O
3
(601 g) so Al is limiting reagent
3.9

Use limiting reagent (Al) to calculate amount of product that can be formed.
g Al mol Al
2Al + Fe
2
O
3 mol Al
2
O
3
Al
2
O
3
+ 2Fe g Al
2
O
3
124 g Al x
1 mol Al
27.0 g Al x
1 mol Al
2
O
3
2 mol Al x
102. g Al
2
O
3
1 mol Al
2
O
3
= 234 g Al
2
O
3
3.9

• Empirical Formula
The formula of a compound that expresses the smallest whole number ratio of the atoms present.
Ionic formula are always empirical formula
• Molecular Formula
The formula that states the actual number of each kind of atom found in one molecule of the compound.

the true or actual ratio of the atoms in a compound the simplest whole number ratio of the atoms in a compound
Example
C
6
H
12
O
6
CH
2
O

1. What is the empirical formula for
C
4
H
8
?
A ) C
2
H
4
B) CH
2
C) CH
2. What is the empirical formula for
C
8
H
14
?
A ) C
4
H
7
B) C
6
H
12
C) C
8
H
14
37

Just find the lowest whole number ratio a nd
It is not just the ratio of atoms, it is also the ratio of moles of atoms

In 1 mole of CO
2 there is 1 mole of carbon and 2 moles of oxygen
In one molecule of CO
2 there is 1 atom of C and 2 atoms of O

1. Find the empirical formula of a compound that contains 42 g of nitrogen and 9 g of hydrogen.
(Ar N = 14 Ar H = 1)

2. Find the empirical formula of a compound containing 20 g of calcium, 6 g of carbon and 24 g of oxygen. (Ar Ca = 20 Ar C = 12
Ar O = 16)

Convert the grams to mol for each element
Write the number of mol as a subscript in a chemical formula
Divide each number by the lowest number.
Multiply the result to get rid of any fractions.

• 1. – Convert the grams to mol for each element
N = mass = 42 g = 3 mol
Molar mass 14 g/mol
H = mass = 9 g = 9 mol
Molar mass
1 g/mol

Write the number of mol as a subscript in a chemical formula
3 mol of N
- 9 mol of H
-
N
3
H
9
Divide each number by the lowest number.
NH
3

1.
4. Calculate the empirical formula of a compound composed of 37 % C, 16 % H, and
47 % N. (Ar C = 12 Ar H = 1 Ar N = 14)
Pretend that you have a 100 gram sample of the compound.
2. change the % to grams
3.
4.
5.
Convert the grams to mol for each element
Write the number of mol as a subscript in a chemical formula
Divide each number by the lowest number.
6. Multiply the result to get rid of any fractions.

• Calculate the empirical formula of a compound composed of 37 % C, 16 % H, and 47 %N.
• Assume 100 g so
• 37 g C = 3.1 mol C
12 g/mol
• 16 g H = 16 mol H
1 g H
• 47 g N = 3.4 mole N
14 g N

• 3.1 mol C
• 16 mol H
• 3.4 mol N
•C
3.1
H
16
N
3.4
If we divide all of these by the smallest one It will give us the empirical formula

• The ratio is 3.1 mol C = 1 mol C
3.1 mol C 1 mol C
• The ratio is 16 mol H = 5 mol H
3.1 mol C 1 mol C
• The ratio is 3.4 mol N = 1 mol C
3.1 mol C 1 mol C
• C
1
H
5
N
1 is the empirical formula or
CH
5
N

6. Caffeine has a molar mass of 194 g. what is its molecular formula?

• Find x if x
 molar empirical mass formula mass
2 X
194 g
97 g
C
4
H
5
N
2
O
1
C
8
H
10
N
4
O
2
= 2

• A compound is known to be composed of
71 % Cl, 25 % C and 4 % H. Its molar mass is known (from gas density) is known to be
98.96 g. What is its molecular formula?