Chemical Equations
Formulae, names, equations,
moles and stoichiometry
Learning objectives
 Apply conservation of matter to balance
chemical equations
 Apply the mole concept to chemical
reactions
 Use chemical equations to predict molar
quantities of reactants and products
The chemical equation
aA + bB = cC + dD
coefficient
Reactant
side
ELEMENT or
COMPOUND
Product
side
The Law of Conservation of Matter states that matter
is neither created nor destroyed
All the atoms on the left must be the same as those
on the right
Chemical book-keeping
 The key to writing correct equations is to ask
the question, “Have I gained or lost any
atoms?”
 Another thing is to put down the correct
formula for each reactant or product
 In the reaction of hydrogen with oxygen to produce
water, the reactants are the elements H2 and O2,
and the product is H2O
The big number
multiplies every
atom after it
 Count the atoms: 4 H and 2 O
The subscript
only multiplies
the atom before
it
4 H and 2 O
Balance the equations
 A method of trial and error
 CH4 + O2 = CO2 + H2O
Try these
 C3H8 + O2 = CO2 + H2O
 N2 + H2 = NH3
Working with equations
 Predict how much product is obtained from
given amount of reactant
 Predict how much reactant is needed to give
required amount of product
 Predict how much of one reactant is
required to give optimum result with given
amount of another reactant
Relating moles, masses and
molecules
The roadmap


Equations are in moles, but we measure in grams
Three conversions required:
1. Must convert grams A to moles A using molar mass
2. Use coefficients in equation to get moles B from moles
A
3. Convert moles B to grams B using molar mass
Mass A
Mass/molar mass
Moles A
Mole:mole ratio
Moles B
Moles x molar
mass
Mass B
Types of problems:
Moles A → moles B
aA + bB = cC + dD
 Mole:mole ratio:
 a mol A ≡ b mol B
moles B
moles A
Mass A
Mass/molar mass
Moles A
Mole:mole ratio

b
mol B / mol A
a
Moles B
Moles x molar
mass
Mass B
Moles A → mass B
1. Convert moles A → moles B:
Mole:mole ratio:
moles B

moles A
b
mol B / mol A
a
2. Convert moles B → mass B using molar mass B
Mass A
Mass/molar mass
Moles A
Mole:mole ratio
Moles B
Moles x molar
mass
Mass B
Mass A → mass B
1. Mass A → moles A using molar mass A
2. Moles A → moles B using mole:mole ratio
3. Moles B → mass B using molar mass B
Mass A
Mass/molar mass
Moles A
Mole:mole ratio
Moles B
Moles x molar
mass
Mass B
Summary of stoichiometry problems

Maximum of three conversions required
1. Must convert grams A to moles A using molar mass
2. Use coefficients in equation to get moles B from
moles A
3. Convert moles B to grams B using molar mass

Maximum of three pieces of information
required
1. Molar mass of given substance (maybe)
2. Molar mass of target substance (maybe)
3. Balanced chemical equation (always)
 Molar mass Cl2 = 35.5 x 2 = 71.0 g/mol
 Molar mass NaOH = 23.00 + 16.00 + 1.01 = 40.01 g/mol
Work this example
 CH4 + 2O2 = CO2 + 2H2O
 What mass of CO2 is produced by the
complete combustion of 16 g of CH4
 Atomic weight H = 1, C = 12, O = 16
44 g