The Mole
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Revision from S.G.
1 mole is the formula mass in grams.
Concentration of solutions is measured in mol/l.
We use moles to Balance equations.
Example – Calculate the mass of 0.5 moles of
Water.
• Formula mass = H2O = 2 + 16 = 18
• 1 mole = 18 g, 0.5 moles = 0.5 x 18g = 9g
Avagadro’s Constant (L)
1 mole of any substance contains the same
amount of “ elementary entities” as 12g ( 1
mole) of C – 12.
“ Elementary entities” can be; atoms,
molecules, ions, units, protons, electrons
etc.
Avagadro’s constant = 6.02 x 10 23 mol/l.
Example
• Water – H2O
• How many molecules are in 1 mole – 18g- of
water?
• 1 x L = 6.02 x 10 23 molecules.
• How many atoms in 1 mole – 18g – of water?
• Each molecule is made of 2 H and 1 O atoms – a
total of 3 atoms.
• 1 mole of water contains 3 x L = 3 x 6.02 x 10 23.
= 1.8 x 10 24 atoms
Water cnt.
• How many molecules are there in 0.5 moles of
water?
• 1 mole = 1 x L
• 0.5 moles = 1.5 x l x L = 0.5 x1 x 6.02 x 10 23.
» = 3.01 x 10 23 molecules
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How many atoms are there in 0.1 moles of water?
In 1 mole there are 3 x L atoms.
In 0.1 moles – 0.1 x 3 x L
= 0.1 x 3 x 6.02 x 10 23 = 1.8 x 10 23 atoms
More examples
• How many protons are there in 1 mole of carbon?
• In 1 atoms of Carbon there are 6 protons( atomic
number)
• In 1 mole there will be 6 x L protons
= 6 x 6.02 x 10 23 = 3.6 x 10 24 protons.
How many protons are there in 0.1 moles of Carbon?
1 mole = 6 x L
0.1 moles = 0.1 x 6 x L
0.1 x 6 x 6.02 x 10 23 = 3.6 x 10 23 protons.
Molar Volume
• The average Molar Volume of a gas can be
taken as 22.4 l – at the same standard
temperature and atmospheric pressure.
• We can calculate molar volume using the
following:
M
D
V
M = mass , D = Density, V = Volume
Worked Examples!
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Data:
Mass of empty flask = 107.49g
Mass of flask + Carbon dioxide = 108.37g
Volume of flask = 480 cm3.
Calculate – Mass of CO2 = 108.37 –
107.49= 0.88g.
• 0.88g of CO2 has a volume 0f 480 cm3.
The Answer!
• What about 1 mole?
• I mole of CO2 = 44g
• 44g has a volume of: 44/0.88 x 480
= 24,000 cm3
The molar volume of Carbon dioxide
is 24,000 cm3 or 22.4 litres.
Calculating Density
• Example
• Calculate the density in g /l of Neon, molar
volume – 24l.
• D = M/V
• D = 20.2/24
• D = 0.84 g/l
Density cnt
• Calculate the density of Ammonia gas in
g/cm3, molar volume 24 l.
• D = M/V
• D = 17/24
• D =0.71g /l or 0.00071g / cm3
• Calculate the molar volume of Argon if it
has a density of 0.0018g cm3
• V = M/D
• V = 40/0.0018
• V = 22, 222cm3 or 22.2 l
Calculations using gas volumes
• Calculate the volume and composition of the
residual gas mixture when 20 cm3 of methane is
burned completely in 100cm3 of Oxygen.
• CH4 + 2O2 —> CO2 + 2H2O
• 1 mole : 2 moles 1 mole : 2 moles
• 20 cm3: 40 cm3
20 cm3: 40 cm3
• The residual gas mixture will contain:
• 20 cm3 CO2, 40 cm3 H2O and 60 cm3 un reacted
oxygen.( The Oxygen is in excess!)