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Topic 1.2

AMOUNT OF SUBSTANCE

The mole
Reacting masses and atom economy
Solutions and titrations
The ideal gas equation
Empirical and molecular formulae
Ionic equations











Mill Hill County High School









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THE MOLE

Since atoms are so small, any sensible laboratory quantity of substance must contain a huge
number of atoms:
1 litre of water contains 3.3 x 1025 molecules.
1 gram of magnesium contains 2.5 x 1022 atoms.
100 cm3 of oxygen contains 2.5 x 1021molecules.

Such numbers are not…

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The mass of one mole of a substance is known as its molar mass, and has units of gmol1. It
must be distinguished from relative atomic/molecular/formula mass, which is a ratio and hence
has no units, although both have the same numerical value.

The symbol for molar mass of compounds…

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REACTING MASSES

It is possible to use the relationship moles = mass/mr to deduce the masses of reactants and
products that will react with each other.

When performing calculations involving reacting masses, there are two main points which must
be taken into account:

The total combined mass of the reactants…

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ii) reactions which do not go to completion

Many inorganic reactions go to completion. Reactions which go to completion are said to be
quantitative. It is because the reactions go to completion that the substances can be analysed
in this way.

Some reactions, however, particularly organic reactions, do not go…

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ATOM ECONOMY

When we carry out a chemical reaction in order to make a product, we often make other
products, called byproducts, as well.

Eg In the production of NaOH from NaCl the following reaction takes place:

2NaCl + 2H2O 2NaOH + H2 + Cl2

The atom economy of a…

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SOLUTIONS

A solution is a homogeneous mixture of two or more substances in which the proportions of
the substances are identical throughout the mixture.

The major component of a solution is called the solvent and the minor components are called
the solutes. In most cases water is the solvent.

The…

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The volume of one solution required to react with a known volume of another can be deduced
from the above relationships and knowledge of the relevant chemical equation. Remember it is
moles which react in the ratio shown, so all quantities must be converted to moles before the
comparison can…

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GASES


The volume occupied by a gas depends on a number of factors:

i) the temperature: the hotter the gas, the faster the particles are moving and the
more space they will occupy
ii) the pressure: the higher the pressure, the more compressed the gas will be and the
less…

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SUMMARY ­ USING MOLES

Using the four relationships described, it is possible to calculate the amount of any substance in
a chemical reaction provided that the chemical equation is known and the amount of one of the
reacting species is also known. The procedure is summarised in the table below:…

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