Complete Chemistry
STOICHIOMETRY
IGCSE NOTES (Updated Syllabus
Version)
The Avogadro constant
When you are carrying out a chemical reaction, you are never using one atom or
molecule, or even just a few. In fact, you will be using huge numbers of them.
Take, for example, the reaction between hydrogen and oxygen gas to form water:
2H2(g) + O2(g) → 2H2O(l)
If you were producing 1 litre (1 dm3) of water, you would actually have
3.34 × 1025 molecules of H2O.
Numbers this big can be difficult to compare and use. Therefore, it is more
appropriate to refer to the amount of substance using a unit called a mole
1 mole contains exactly 6.02214076 × 1023 elementary entities. An elementary
entity may be an atom, a molecule, an ion, an electron or any other particle or
specified group of particles. This number is referred to as the
Avogadro constant can be expressed in standard form as 6.02 × 10 23 . It is called a
‘constant’ as the number never changes – it is constant.
1 mole (symbol ‘mol’) of a substance is related to its relative atomic mass (Ar) or
relative molecular mass (Mr) as 1 mole of the substance is the Ar or Mr expressed
in grams.
Molar gas volume
Avogadro discovered that under the same temperature and pressure conditions,
equal volumes of different gases would contain an equal number of molecules.
His hypothesis has been further refined and we can now say with some clarity
that:
, 1 mole of a gas occupies a volume of 24 dm 3 (or 24 000 cm 3 )
at room temperature and pressure (r.t.p.).
r.t.p. stands for room temperature and pressure, which means measurements are
made at the typical conditions found in a laboratory. More precisely, this stands
for 25°C at 1 atmosphere of pressure.
The number of moles of a gas at r.t.p. can be calculated using the formula:
gas volume (v )
Number of moles (n)= 3
24 dm
Units of volume
Volumes of fluids (liquids and gases) can be measured using a number of different
units: cubic centimetres (cm 3 ), cubic decimetres (dm 3 ) or litres (l).
You need to be able to convert between cm 3 and dm 3 . To convert from dm 3 to
cm 3 , you multiply by 1000. To convert cm 3 to dm 3 , you divide by 1000.
Mole calculations with mass
The amount of a substance in moles (mol) can be calculated from its mass using
the formula:
Mass
Mol=
Mr
The molar mass is the mass of one mole, so it is the same as the relative
molecular mass, M r , expressed in grams per mole.
STOICHIOMETRY
IGCSE NOTES (Updated Syllabus
Version)
The Avogadro constant
When you are carrying out a chemical reaction, you are never using one atom or
molecule, or even just a few. In fact, you will be using huge numbers of them.
Take, for example, the reaction between hydrogen and oxygen gas to form water:
2H2(g) + O2(g) → 2H2O(l)
If you were producing 1 litre (1 dm3) of water, you would actually have
3.34 × 1025 molecules of H2O.
Numbers this big can be difficult to compare and use. Therefore, it is more
appropriate to refer to the amount of substance using a unit called a mole
1 mole contains exactly 6.02214076 × 1023 elementary entities. An elementary
entity may be an atom, a molecule, an ion, an electron or any other particle or
specified group of particles. This number is referred to as the
Avogadro constant can be expressed in standard form as 6.02 × 10 23 . It is called a
‘constant’ as the number never changes – it is constant.
1 mole (symbol ‘mol’) of a substance is related to its relative atomic mass (Ar) or
relative molecular mass (Mr) as 1 mole of the substance is the Ar or Mr expressed
in grams.
Molar gas volume
Avogadro discovered that under the same temperature and pressure conditions,
equal volumes of different gases would contain an equal number of molecules.
His hypothesis has been further refined and we can now say with some clarity
that:
, 1 mole of a gas occupies a volume of 24 dm 3 (or 24 000 cm 3 )
at room temperature and pressure (r.t.p.).
r.t.p. stands for room temperature and pressure, which means measurements are
made at the typical conditions found in a laboratory. More precisely, this stands
for 25°C at 1 atmosphere of pressure.
The number of moles of a gas at r.t.p. can be calculated using the formula:
gas volume (v )
Number of moles (n)= 3
24 dm
Units of volume
Volumes of fluids (liquids and gases) can be measured using a number of different
units: cubic centimetres (cm 3 ), cubic decimetres (dm 3 ) or litres (l).
You need to be able to convert between cm 3 and dm 3 . To convert from dm 3 to
cm 3 , you multiply by 1000. To convert cm 3 to dm 3 , you divide by 1000.
Mole calculations with mass
The amount of a substance in moles (mol) can be calculated from its mass using
the formula:
Mass
Mol=
Mr
The molar mass is the mass of one mole, so it is the same as the relative
molecular mass, M r , expressed in grams per mole.
