The document defines key concepts related to the mole including: - The mole is a unit that represents 6.022x10^23 particles and allows chemists to conveniently work with substances at a macroscopic scale. - One mole of any substance has a mass in grams numerically equal to its molar mass. - Common calculations involve determining moles, mass, particles, or volume of a substance using its molar mass and amount in moles. - The molar volume of a gas is 22.4L at STP allowing for volume calculations.

1. The Mole
Section 3.3

2. The Mole
• The mole is simply a chemist’s measurement for the
amount of a substance.
• It is a very large number (6 x 1023) as it ‘counts’ very
small ‘objects’ such as atoms and molecules which
have a very small masses.
• This rather strange number was not selected
deliberately – instead, it is the number of atoms in a
sample of any element that has a mass in grams that is
numerically equal to the elements atomic mass.
• It allows chemists to work with masses in the lab which
are feasible.

3. The Mole Definition
A mole of any substance is defined as the
amount of the substance that contains as many
particles as 12 g of carbon – 12.

4. (i)
(ii)
How many molecules are there in 0.5 mols of chlorine gas?
How many atoms are there in 2 mols of water?
(iii)
How many electrons are there in 1.5 mols of Calcium?
(i)
1 mol of Cl2
6 1023 molecules
0.5 mols of Cl2
(ii)
0.5 6 1023
1 mol of H2 0 6 1023 molecules
[Cl2 is composed of molecules]
3 1023 molecules
[H2 O is composed of molecules]
2mols of H2 0 2 6 1023 1.2 1024 molecules
Each molecule of water contains 3 atoms
1.2 1024 molecules contains 3 1.2 1024
(ii)
3.6 1024 atoms.
1 mol of Ca 6 1023 atoms [Ca is composed of atoms]
1.5 mols of H2 0 1.5 6 1023
9 1023 atoms
Each atom of Calcium contains 20 electrons
9 1023 atoms contain 23 9 1024
2.07 1025 electrons.

5. Molar Volume
• One mole of any gas should occupy the same volume
as one mole of any other gas at the same conditions
of temperature and pressure.
• Gases are often compared at STP, standard
temperature and pressure.
Standard pressure = 101325 Pa
Standard temperature = 273 K
• Molar Volume = 22.4 L = 22,400 cm3 = 2.24 x 10–2 m3

6. (i)
What is the volume in litres at STP of 0.01 mols of methane (CH4 ) gas?
(ii)
How many mols are there in 280 cm3 of fluorine gas at STP?
(i)
1 mol of CH4
22400 cm3
0.01mols of CH4
(ii)
0.01 22400 224 cm3
1 mol of F2 22400 cm3
1
mols of F2 = 1cm3
22400
1
280 mols of F2 = 280cm3
22400
0.0125 mols of F2 280cm3

7. Relative Molecular Mass
The relative molecular mass is the average mass
of a molecule relative to one twelfth the mass of
the carbon 12 atom.
The relative molecular mass has no units as it is
the ratio of two masses.

8. (i)
Calculate the relative molecular mass of (a) sulfuric acid, (H2 SO4 ) (b) Oxygen, (O2 ).
(i)
(a)
H2 SO4
(b)
O2
2(1) 32 4(16) 98
2(16) 32

9. Molar Mass
• The molar mass of a substance is the mass in grams
of one mole of the substance.
• The molar mass has the same numerical value as its
relative molecular mass, but its units are grams (g).
Substance
Relative molecular mass Molar mass
Sulfuric aicd, H2 SO4
98
98 g
Glucose, C6H12 O6
180
180 g
Oxygen, O2
32
32 g
Sulfur dioxide, SO2
64
64 g
1 mol of carbon, 12 g

10. (i)
How many moles are there in 990 g of cabon dioxide (CO2 )
(ii)
The daily intake of calcium for an adult is 800 mg, how many moles of calcium is this?
(i)
1 mol of CO2
44 g
1
mols of CO2 1g
44
1
990 mols of CO2 990 g
44
22.5 mols of CO2 990 g
(ii)
1 mol of Ca 40 g
1
mols of Ca 1g
40
1
800 10 3 mols of Ca 800 10 3 g
40
0.02 mols of Ca 800 10 3 g
[800mg 800 10 3 g]

11. More Mole Calculations
Volume of X
Number of
particles of X
(if a gas at STP)
in litres at STP
Moles of X
x molar mass
÷ molar mass
Mass of X in g
Notice that there
is no direct link
from particles to
grams, you must
first convert to
moles. Likewise
going from
volume to mass.

12. What is the mass of one atom of calcium?
1 mol of Ca 40 g
1 mol of Ca 6 1023 atoms
6 1023 atoms 40 g
1 atom
40
6 1023
6.67 10
23
g

13. The Nissan Micra 1.5 Diesel SVE is quoted to have a CO2 emission figure of 120 g / km.
For a 40 km round trip to work calculate:
(i)
(ii)
(iii)
the mass of CO2 produced.
the number of moles of CO2 produced.
the volume of CO2 produced at room temperature and pressure.
[Molar volume at room temperature and pressure 24.0 litres]
(i)
120 40 4800 km
(ii)
no. of mols
(iii)
1mol of CO2
actual mass
molecular mass
4800
no. of mols
mols
44
no. of mols 109.09 mols
24.0 L
109.09 mols of CO2
109.09 mols of CO2
109.09 24L
2594.16 L

14. How many iron atoms should be consumed daily to meet the recommended daily intake
of iron in the diet of 0.014 g?
no. of mols
acutalmass
molecular mass
0.014
56
no. of mols 2.5 10 4 mols
no. of mols
1mol 6 10 3 atoms
2.5 10 4 mols 2.5 10
4
2.5 10 4 mols 1.5 10
0
6 10 3 atoms
atoms
[H2007, Q4 (e)]