The Mole

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The Mole
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What is a mole??? As we have seen in previous
chapters the masses of atoms are very small e.g.
mass of proton 1.67 x 10-27kg mass of
neutron 1.67 x 10-27 kg mass of electron
negligible Subsequently it is not possible to
count individual atoms or molecules directly
!!! Amedeo Avogadro was an Italian chemist who
devised a method for counting atoms and molecules
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1 atom of Mg 24 (Ar) 1 atom of C 12 (Ar) So
can say 100 Magnesium atoms is twice as heavy as
100 Carbon atoms Therefore if we have a piece of
Mg that is twice the weight of a piece of carbon
can say the two amounts contain equal numbers of
atoms Similarly if we had 2g of Mg and 1g of C
they would contain the same number of atoms
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1 atom of carbon 12 Ar 1 atom of Hydrogen 1
Ar 1g of hydrogen atoms and 12 g of carbon atoms
both contain the same number of atoms So, if we
were to weigh out elements in the same
proportions as their relative atomic masses, we
will always be dealing with the same number of
atoms of these elements.
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If we take the relative atomic mass in grams of a
number of elements All of these amounts can
be said to contain the same number of atoms -
shown by Avogadro to be L 6 x 1023
mol-1 Avogadros Constant (Number)
12g 24g 63.5g 108g
Carbon Magnesium Copper Silver
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Avogadros Constant ( 6 x 1023 ) is the number of
atoms of carbon in 12g of the carbon 12
isotope One mole of a substance is the amount of
that substance which contains 6 x 1023 particles
(atoms, ions, molecules) of that substance
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• Points to Note
• Symbol for mole mol
• 12g carbon 6 x 1023 atoms of carbon 1 mole
  of
• carbon atoms
• Similarly,
• 48g of Mg 2 moles of magnesium
• atoms
• Must specify what particle amount refers to
• 1 mol of Hydrogen atoms/1 mol of H ( 1g H
  atoms)
• 1 mol of Hydrogen molecules/ 1 mol of H2 ( 2g
  H2
• (molecules)

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• 1. Calculations Involving the Mole
• Converting Moles to Grams
• 2. Converting Grams to Moles
• 3. Calculations involving Avogadros Number
• 4. More complicated problems

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Molar Mass is the mass (in grams) of one
mole of a substance. Molar mass of
carbon is 12g/mol Molar mass of magnesium is
24g/mol
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Converting Moles to Grams Mass of 1 mole of an
element Relative Atomic Mass in
grams e.g. 1 mole of Na 23 g 1 mole of Mg
24g 1 mole of copper ??? 1 mole of iron
??? 1 mole of zinc ???
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The relative molecular mass (Mr) of a compound is
the sum of the relative atomic masses of all the
atoms in a molecule of the compound. The
relative molecular mass (Mr)of a compound may be
defined as the mass of one molecule of that
compound compared with one twelfth of the mass of
the carbon-12 isotope Mass of 1 mole of
Relative Molecular Mass a
compound in grams
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Converting Moles to Grams
Mass (in grams) Relative x Number
of Molecular Mass Moles
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Example 1 What is the mass of a mole of helium
atoms? Solution Example 2 What is the mass of
three moles of aluminium atoms? Solution
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Example 3 What is the mass of a mole of nitrogen
molecules? Solution Example 4 What is the
relative molecular mass of water? What is mass of
a mole of water? Solution
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Example 5 What is the mass of one mole of
sulphate ions, SO42- ? Solution Example
6 What is the mass of one mole of aluminium
sulphate Al2(SO4)3 ? Solution
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Example 7 What is the mass of 0.2 moles of CaCO3
? Solution
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Converting Grams To Moles
No. of Moles of Mass of Element
an element Relative Atomic
Mass No. of Moles of Mass of
Compound Compound Relative
Molecular Mass No. of Moles
of Mass of Substance Substance
Mass of 1 mole of Substance
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Example 1 How many moles are there in 1g of
Helium atoms? Solution Example 2 How many
moles of atoms are there in 10g of
Helium? Solution
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Example 3 How many moles of water are in 90g of
water Solution Example 4 How many moles of
sulphuric acid, H2SO4, are there in 12.25g of
sulphuric acid? Solution
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Calculations Involving Avogadros
Number Remember Avogadro's Number represents
the number of carbon atoms in 12g of the
carbon-12 isotope 6 x 1023 It is
important to remember that the atomic mass of any
other element, expressed in grams, will also
contain the same number of atoms. A mole is the
amount of a substance which contains 6 x 1023
particles atoms, ions or molecules, of that
substance.
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Calculations involving Avogadros number may be
divided into four types 1) Converting Moles to
number of Atoms or Molecules 2)
Converting Grams to number of Atoms or
Molecules 3) Converting Numbers of Atoms or
Molecules to Moles 4) Converting
Numbers of Atoms or Molecules to Grams
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Key to solving these problems
1 Mole of a substance contains 6 x 1023 particles
of that substance
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• Converting Moles to number of Atoms or Molecules
• Atoms
• How many atoms of sodium are present in 0.25
  moles of sodium? (composed of Na atoms)
• Answer
• b) Molecules
• How many molecules are present in 5 moles of
  water
• (Composed of water molecules)
• Answer

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c) How many i) molecules ii) atoms are present in
1.5 moles of water (composed of water
molecules) Answer
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• 2. Converting Grams to number of Atoms or
  Molecules
• Steps Involved
• 1) Find number of moles
• moles mass/molar mass
• 2) Find number of molecules(atoms)
• molecules (atoms) moles x 6 x 1023
• 3) Find number of atoms (Compound)
• atoms no. of atoms x no. of molecules

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• How many atoms are there in 28.75g of sodium
• (composed of Na atoms)
• Answer

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b) How many molecules are contained in 4.9g of
sulphuric acid? (composed of molecules of
sulphuric acid)
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c) How many atoms are contained in 34g of
ammonia? (Composed of molecules of ammonia)
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3. Converting numbers of Atoms or Molecules to
Moles Example How many moles of atoms
are present in 5 x 1024 atoms of
carbon? Answer Note Use brackets in
calculations
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4. Converting numbers of Atoms or Molecules to
Grams Example 1 How many grams of
zinc are present in 3 x 1020 atoms of
zinc? Answer
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Example 2 What is the mass (in grams) of 3 x
1022 molecules of ethanol C2H5OH
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More Complicated Problems Example 1 How many
moles of Al3 are in 3 moles of Al2(SO)4? Answer
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Example 2 How many moles of oxygen atoms are
there in 6.3g of nitric acid, HNO3 Answer
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Example 3 What mass of magnesium contains the
same number of atoms as 7g of iron? Answer