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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 convenient to work with, so it is necessary to find a unit of "amount"
which corresponds better to the sort of quantities of substance normally being measured.…read more

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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 or molecular elements is mr. The symbol for molar
mass of atoms is ar.…read more

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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 must be the same as the total combined mass
of the products. This is known as the law of conservation of mass.…read more

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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 to completion. It is possible
to calculate the percentage yield of product by using the following equation:
% yield = amount of product formed x 100
maximum amount of product possible
Eg 2.0 g of ethanol (C2H5OH) is oxidised to ethanoic acid (CH3COOH).…read more

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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 reaction is the percentage of the total mass of reactants that can, in
theory, be converted into the desired product.…read more

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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 amount of solute present in a fixed quantity of solvent or solution is called the
concentration of the solution.…read more

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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 be made.
The quantitative investigation of chemical reactions by comparing reacting volumes is known as
volumetric analysis. The procedure by which reacting volumes are determined is known as a
titration.…read more

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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 space it will occupy
iii) the amount of gas: the more gas particles there are, the more space they will
The volume occupied by a gas does not depend on what gas it is,…read more

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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:
These relationships are frequently used in practical chemistry.…read more


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