Download 1.1.4 Amount of Substance / The Mole

1.1.4 Amount of Substance / The Mole
You will be able to…
• Explain the terms amount of substance, mole and
the Avogadro constant.
• Define and use the term molar mass.
• Carry out calculations involving masses using the
amount of substance in moles.
The Mole
Derived from the Greek
word Mole meaning
large heap
The Mole
• The chemical mole is a heap of atoms or molecules
Dalton
• In 1808 Dalton proposed that the atoms of each
element had a characteristic atomic weight and it
was atoms that were the combining units in
chemical reactions to make compounds
• Dalton had no method of measuring atomic weights
unambiguously, so made the incorrect assumption
that in the most common compound between two
elements, there was one atom of each element e,g
water was made from one atom of hydrogen and
one of oxygen, oxygen gas contained only one atom
of oxygen
Gay-Lussac
• Gay-Lussac studied the chemical reactions of gases
and found that the ratios of volumes of the reacting
gases were small integer numbers e.g two volumes
of hydrogen react with one of oxygen to produce
two volumes of water
Lorenzo
Romano
Amadeo
Carlo
Avogadro
Avogadro
• In 1811, Avogadro clearly drew the distinction between a
molecule and an atom
• He pointed out that Dalton had confused the concepts of
atoms and molecules. The Dalton ‘atom’ of Oxygen gas was in
reality a molecule containing two atoms
• Avogadro also suggested that ‘equal volumes of all gases at
the same temperature and pressure contain the same number
of molecules’ This is known as Avogadro’s Principle
• Avogadro was Italian so everybody ignored his work until
Stanisalo Cannizarro showed how it could be used to
determine molar and indirectly atomic mass at the Karlsuhe
Conference 1860
The Mole Argument
• From atomic masses one atom of magnesium is twice as heavy as
one atom of carbon (Mg = 24 and C = 12). Therefore…
• If one atom of Mg is twice as heavy as one atom of C then 1000
atoms of Mg are twice as heavy as 1000 atoms of C etc.
• A piece of Mg twice as heavy as a piece of C must contain the same
number of atoms e.g 2 tonnes of Mg contains the same number of
atoms as 1 tonne of Carbon
12g
C
24g
Mg
56g
Fe
108g
Ag
197g
Au
Extend this to molecules
• Molecules are made up of atoms in different ratios
• The relative molecular mass of a molecule in grams contains
one mole of that specific molecule
16g
CH4
17g
NH3
18g
H2O
44g
CO2
34g
H2S
Definitions
One Mole is the amount of substance containing the
same number of units there are in exactly 12g of 12C
The substance must be specified and may be atoms,
molecules, ions, crystals grains of sand, flies sheep,
trucks… anything!
Avogadro’s Constant is the number of units in a mole
It is given the symbol L and is 6.0221415 x 1023
So What?
• What is difficult to grasp is the size of Avogadro’s
number, 6.0221415 x 1023 that is…
602 214 150 000 000 000 000 000
Basically if you can see a particle of it with your eye
you wont be able to gather a mole of it on this earth
Mole Calculations (I)
The number of moles of a substance is defined as the
amount of substance, n. To calculate n just divide
the mass of the substance, m, by the molar mass, M
n =
mass
Molar mass
=
m
M
=
g
g mol -1
= mol
Mole Calculations (II)
To calculate the number of atoms or molecules = n x L
Number of atoms
or molecules
=
mass
Molar mass
x Avogadro’s Constant
= mL = nL
M
=
g
g mol -1
x
mol -1
Mole Problems
Relative atomic masses: H = 1; O = 16; Na = 23; Cl = 35.5;
Ca = 40; Cu = 64
1.
2.
3.
4.
5.
6.
What is the mass of 4 mol of sodium chloride, NaCl?
How many moles is 37g of calcium hydroxide,
Ca(OH)2?
How many moles is 1kg of calcium?
What is the mass of 0.125 mol of copper oxide, CuO?
0.1 mol of a substance weighs 4g. What is the weight
of 1 mole?
0.004 mol of a substance weighs 1g. What is the
relative formula mass of the compound?
n =m
M