The Mole
Chemistry 8(A)
The Mole
Lesson Objectives
• Define the mole
• Use the concept of the mole
The Mole
• Mole – unit of measurement that is equal to 6.02 × 1023
particles
– Particle – a very small piece of matter
• Examples of different types of particles include atoms, ions, electrons,
molecules and formula units
• Avogadro’s number = 6.02 × 1023 particles
– Avogadro’s number is based on the number of atoms in 12 g of
carbon-12
• Mole is abbreviated as mol
Mass and Moles
• A mole is a unit based on number of particles not mass
• Each element has a unique mass
• Substances with the same number of moles can have
different masses
Molar Mass
• Molar mass – mass of one
mole of pure substance
-
Units of g/mol
Ex) Molar mass of Sr
• Molar mass of a single
element is numerically
equal to its atomic mass
87.62 g/mol
Subscript Review
• Subscripts indicate the
number of atoms of each
element in the formula
Ex) H2O2
2H
2O
• Absence of a subscript
indicates there’s only one
atom
H
O
O
H
Ex) NH3
1N
3H
N
H
H
H
Parentheses Review
• Subscripts placed outside
parentheses apply to all the
elements inside the
parentheses
• Multiply any subscripts inside
the parentheses by the
subscript outside to find the
total number of atoms of each
element
Ex) Mg(OH)2
[
]–
2+ [
1 Mg
2O
2H
]–
Formula Mass and Molar Mass
• Formula mass – total of
the atomic masses of
the atoms in a formula
• Molar mass of a
compound is
numerically equal to the
formula mass
Ex) Formula mass of MgBr2
1 atom Mg × 24.305 = 24.305
2 atoms Br × 79.904 = 159.808
+
184.113
Molar Masses of Compounds
• Considering the number
of moles of each
element present, the
molar mass of a
compound equals the
sum of the molar masses
of its elements
• Subscripts in the formula
indicate the number of
moles of atoms of each
element in one mole of
the compound
Ex) Molar mass of MgBr2
1 mol Mg × 24.305 g/mol Mg = 24.305 g Mg
2 mol Br × 79.904 g/mol Br
= 159.808 g Br
+
184.113 g/mol MgBr2
Calculating Molar Masses
1. Multiply the molar
mass of each element
by the number of
moles of that element
in the compound
2. Add the masses of all
the elements together
Ex) Molar mass of
Mg(OH)2
1 Mg = 24.305 × 1 = 24.305
2 O = 15.999 × 2 = 31.998
2 H = 1.0079 × 2 = 2.0158
+
58.319 g/mol Mg(OH)2
The Mole
Lesson Objectives
• Define the mole
• Use the concept of the mole
– Calculate molar mass