P5
Quantitative Aspects of Formulae and Equations
The mole concept is one of the fundamental concepts of modern chemistry as the use
of this concept enables scientists and science students to investigate the quantitative
relationships in chemical reactions. Formulae and stoichiometric equations are used in
these calculations.
Stoichiometry
The study of quantitative aspects of formulae and equations
Relative atomic mass
The average mass of the atoms present in the naturally
occurring element relative to the mass of an atom of the
carbon-12 isotope
Commonly called atomic weight
Not the mass of an atom of that element, it is a ratio and has
no units.
Relative molecular
mass
The mass of a molecule of the compound relative to the mass
of an atom of the carbon-12 isotope
The sum of the atomic weights of the atoms as given in the
molecular formula
Relative formula
mass
The sum of the atomic weights of the atomic species in the
stated formula of the compound
The empirical formula is used for ionic compounds
Mole
Avogadro’s number of particles (6.02 x 1023)
The quantity of a substance that contains as many atoms, ions
or molecules as there are atoms in exactly 12 grams of the
carbon-12 isotope
Mole of an element
The mass which is numerically equal to the atomic weigh of
the element
Mole of a compound
The mass which is numerically equal to the molecular weight
of the compound
Molar mass
The mass of a mole of the substance
Mole formulae
Where
n = m
M
n = number of moles
m = mass in grams
M = atomic or molecular weight in grams
N = n x 6.02 x 1023
Where
N = number of atoms or molecules
n = number of moles
6.02 x 1023 is Avogadro’s number
Percentage
composition
Mass of the element in one mole of the compound
Mass of one mole of the compound
Empirical formula
Simplest whole number ratio of the atoms of each element in
the compound
Molecular formula
The number of each type of atom in a molecule of the
compound.
Calculating an
empirical formulae
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x 100
Write down the masses of all the elements present in a
given sample of the compound
Convert masses to moles by dividing each mass by its
atomic weight
Divide through by the smallest number of moles to get a
simple ratio
Multiply through by a suitable factor to get whole
numbers
Round off the numbers to get whole numbers and use
these to write the empirical formulae
Chemical equation
Describes the ratio by mass in which substances react or are
formed in a reaction.
Performing a massmass calculation
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Write a balanced chemical equation
Calculate the number of moles of the given substance
Use the stoichiometric coefficients in the chemical
equation to link the number of moles of the given
substances to the number of moles of the required
substance
Use this link to calculate the number of moles of the
required substance
Calculate the mass of the required substance.
Atomic theory
This theory was proposed by John Dalton in 1803 and the four
postulates were
 Matter is composed of small indivisible spheres (atoms)
 All atoms of one elements are identical
 The atoms of different elements are different
 Atoms of different elements combine in simple whole
number ratios.
Law of combining
volumes
Also known as Gay-Lussac’s Law
When measured at constant temperature and pressure, the
volumes of gases taking part in a chemical reaction show
simple whole number ratios to one another
Avogadro’s
hypothesis
When measured at the same temperature and pressure, equal
volumes of gases contain the same number of molecules.
When measured at the same temperature and pressure, equal
numbers of gas molecules occupy the same volume
Percentage yield
The amount of product obtained from a chemical reaction
expressed as a percentage of the amount expected from the
chemical equation.