SCH3U
Miss Richardson

2 shoes = 1 pair

12 eggs = 1 dozen


24 bottles = 1 case
52cards = 1 deck

6.022 x 1023 carbon atoms = 1 mole

The mass of a single atom is incredibly
small.

1811 - Amedeo Avogadro
 determined that any quantity of matter
must contain an enormous number of
atoms, ions, molecules, etc.
 he showed that 12.01g of carbon contains
6.02 x 1023 atoms
 Why is 12.01g a significant quantity?

Further investigations showed consistency:
 A sample of NaCl has an average atomic mass of
58.44 amu (22.99 + 35.45)
 There are 6.022 x 1023 molecules in 58.44 g of
NaCl

The quantity 6.022 x 1023 is known as
Avogadro’s number (NA) and is equal to 1
mole of a substance.

Eg:
eggs
1 pair of eggs = 2 eggs
1 dozen eggs = 12 eggs
1 mole of eggs = 6.022 x 1023



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monatomic elements = atoms
eg: Fe(s), S(s)
diatomic particles = molecules
eg: O2(g), F2(g), Cl2(l)
molecular compounds = molecules
eg: H2O(l), SO2(g)
ionic compounds = formula units
eg: NaCl(s), LiOH(g)

The number of moles of a substance can be
determined using this formula:
N
n
NA
 Where: n = number of moles.
N = number of particles
NA = Avogadro’s Constant = 6.022 x 1023

Example:
 An airbag contains 3.65 mol of hydrazine (N2H4(l))
▪ How many molecules are there?
N = n x NA
N = 3.65 mol x 6.022 x 1023 molecules
1 mol
= 2.197 300 x 1024 molecules
= 2.20 x 1024 molecules

Example:
 An airbag contains 3.65 mol of hydrazine (N2H4(l))
▪ How many molecules are there?
2.20 x 1024 molecules
▪ How many atoms are there?
2.197 300 x 1024 molecules x 6 atoms
1 molecule
= 1.318 38 x 1025 atoms
= 1.32 x 1025 atoms

When testing a product, chemists need to be
sure the correct number of molecules (or moles
of molecules) of the active ingredient are
present.

To do this, we need a way to relate mass of a
substance to the number of particles present.

molar mass = mass of 1 mole (or 6.022 x 1023
particles) of any substance.

molar mass = mass of 1 mole (or 6.022 x 1023
particles) of any substance.
 Where:
m
M
m = mass of n
a substance (g)
n = #of moles of a substance (mol)
M = molar mass (g/mol)

What is the molar mass (M) of magnesium
chloride, MgCl2?
 Step 1: Find atomic masses on periodic table
MMg = 24.31 g/mol
MCl = 35.45 g/mol
 Step 2: Add atomic masses, accounting for
number of each atom
MMgCl2 = MMg + MCl2
= 24.31 +(2x35.46) = 95.23 g/mol
(m)