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Specification statements
• Explain the terms;– Amount of substance
– Mole (symbol ‘mol’) as the unit for amount of
substance
– Avogadro’s constant NA, as the number of
particles per mole (6.022 x 1023 mol-1)
• Define and use the term ‘molar mass’ (units gmol-1)
as the mass per mole of a substance.
Most
Describe the terms moles and molar mass
Some
Explain how to use moles and molar mass in
calculations
Starter
Look at the substances on show. 12g of Carbon
32 g of Sulphur
27 g of Aluminium
What do they have in common?
Answer
They all contain exactly the same number of
particles. We call this amount 1 Mole.
The Mole
• 1 mole is the amount of substance which
contains as many particles as there are atoms
in exactly 12 grams of 12C.
• This value is 6.022 x 1023 particles.
• For an element it is the Ar in grams
• For example 1 mole of hydrogen atoms has a
mass of exactly 1 gram.
Avogadro’s Constant
• Avogadro said that in 1 mole of any substance
there 6.022x1023 particles. This is called
Avogadro’s constant and is given the symbol
NA. It has the units mol-1.
• Even though there are the same number of
particles in a mole, they have different masses
according to their Ar.
Thinking big
• The Avogadro number is really really big! Lets
look at some other big numbers.
• There are approximately 400 billion stars in a
typical galaxy like the milky way.
• There are approximately 80 billion galaxies in
the universe
• There are approximately 3 x 1023 stars in the
observable universe.
• There are approximately 15 times as many
water molecules in a cup of tea than there are
Molar Mass
This is defined as the mass of 1 mole of a substance. It
has the units gmol-1.
When the number of moles equals 1 then this number
is also equal to;
• Relative atomic mass (Ar) or
• Relative molecular mass (Mr).
Water molecules have Mr = 18.
therefore 1 mole of water has a mass of 18 grams.
The molar mass of water = 18 gmol-1
1 mole of 12C contains exactly 12 grams of carbon atoms.
Which of the statement(s) below is/are correct?
1. 1 mole of oxygen has a mass of exactly 8 grams
2. 1 mole of sodium has a mass of exactly 23 grams
3. 1 mole of chlorine molecules have a mass of exactly 71 grams
4. 1 mole of water molecules have a mass of exactly 10 grams
How would you correct the incorrect statements?
Calculating number of moles
Number of moles (mol) = mass (g)
molar mass (gmol-1)
This equation can be rearranged to calculate mass or molar mass as well as number of moles.
Mass (g)
Mr (gmol-1)
No. of moles
• How many moles of Ca are there in 120g of Ca?
Ar Ca = 40, therefore Mr Ca = 40 gmol-1
number of moles = 120
40
= 3.0 mol
AfL – using whiteboards
Calculate the amount in moles in the following;
1. 32.1 grams of sulfur atoms
2. 50.0 grams of CaCO3
3. 6.35 grams of copper atoms
4. 400 grams of NaOH
5. 69g of Pb
6. 5.30g of Na2CO3
Calculating masses
Worked example
What mass of NaCl contains 10 moles of NaCl particles?
Mr NaCl = 23 + 35.5 = 58.5
therefore Mr of NaCl = 58.5 gmol-1
From the equation on the previous slide ;
Mass = moles x Mr
Mass = 10 mol x 58.5 gmol-1 = 585 grams
AfL – using whiteboards
Calculate the mass of
1. 0.013 moles of Cl2
2. 2.00 moles of SO3
3. 0.25 moles S8
4. 0.15 moles of MgSO4.7H2O
Plenary
• Write your own definitions of ;– The mole
– Molar mass of an element
– Molar mass of a compound
• Compare to statements from mark schemes
• Learn these statements!