
WATER of CRYSTALLISATION CALCULATIONS
hydrated and anhydrous salts
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14.
Other GCSE chemical calculations -
WATER OF CRYSTALLISATION - method of
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Keywords: Quantitative chemistry
calculations How to determine the water of
crystallisation in a salt like compound, hence determine the full formula of the
hydrated salt. How to calculate the % water in a hydrated salt i.e. the
percentage of water of crystallisation in a salt. How to deduce the number of
molecules of water of crystallisation in a salt - fully worked out example
calculations of water of crystallisation. Online practice exam chemistry
CALCULATIONS and solved problems for KS4 Science GCSE/IGCSE CHEMISTRY and basic
starter chemical calculations for A level AS/A2/IB courses. These revision notes
and practice questions on how to do water of crystallisation chemical
calculations and worked examples should prove useful for the new AQA, Edexcel
and OCR GCSE (9–1) chemistry science courses.
Spotted any careless error?
EMAIL query ? comment or request a type of
GCSE calculation not covered?
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See also
14.1
% purity of a product and assay calculations
14.2a
% reaction yield and theoretical yield calculations
and why you can't actually get 100% yield in practice
14.2b atom economy calculations
*
14.3 dilution of solutions calculations
14.5
how
much of a reactant is needed? calculation of quantities required, limiting
reactant quantities
Chemical &
Pharmaceutical Industry Economics & Sustainability, Life Cycle
Assessment, Recycling
14.4 Water of crystallisation
in a crystallised salt
-
Example 14.4.1:
Reminders on calculating formula mass e.g. with
MgSO4.7H2O
-
Relative atomic masses:
Mg = 24, S = 32, O = 16 and H = 1
-
You need to add together the formula mass
of MgSO4 plus the relative mass of seven water molecules.
-
Relative formula mass of water = (2 x 1)
+ 16 = 18
-
Relative formula mass of MgSO4
= 24 + 32 + (4 x 16) = 120
-
Relative mass of seven water molecules =
7 x 18 = 126
-
Relative formula mass of crystals =
MgSO4 + (7 x H2O) = 120 + 126 = 246
-
Example 14.4.2 How to calculate the
theoretical % of water in a hydrated salt
-
eg magnesium sulphate MgSO4.7H2O
'hydrated' salt crystals
-
Relative atomic masses:
Mg = 24, S = 32, O = 16 and H = 1
-
Relative formula mass of crystals =
24 + 32 + (4 x 16) + {7 x (1 + 1 + 16)} = 246
-
Relative mass of seven water molecules =
7 x 18 = 126
-
so % water = 126
x 100 / 246 = 51.2%
-
Example 14.4.3 Determination and
calculation of salt formula containing 'water of
crystallisation'.
-
Some salts,
when crystallised from aqueous solution, incorporate water molecules
into the structure. This is known as 'water of crystallisation', and the
'hydrated' form of the compound.
-
e.g. magnesium sulphate MgSO4.7H2O.
The formula can be determined by a simple experiment (see the copper
sulphate example below).
-
A known mass of the hydrated salt is gently
heated in a crucible until no further water is driven off and the weight
remains constant despite further heating.
-
The % water of
crystallisation and the formula and formula mass of the salt are calculated as follows:
-
Suppose 6.25g of blue
hydrated copper(II) sulphate, CuSO4.xH2O, (x
unknown) was
gently heated in a crucible until the mass remaining was a constant 4.00g.
-
When the mass on
subsequent weighings stays constant, you know all the water of
crystallisation has driven off by the heat.
-
This
is the white anhydrous copper(II) sulphate.
-
The mass of anhydrous
salt = 4.00g, mass of water (of crystallisation) driven off =
6.25-4.00 = 2.25g
-
The % water of
crystallisation in the crystals is 2.25 x 100 / 6.25 = 36%
-
[ Ar values:
Cu=64, S=32, O=16, H=1 ]
-
The mass ratio of CuSO4
: H2O is 4.00 : 2.25 (or 64% : 36%, doesn't matter which
mass ratio you use)
-
To convert from mass
ratio to mole ratio, you divide by the masses molecular/formula mass
Mr of each
'species'
-
Mr CuSO4 = 64
+ 32 + (4x18) = 160 and Mr H2O = 1+1+16 =
18
-
The mole ratio of CuSO4
: H2O is 4.00/160 : 2.25/18
-
which is 0.025 : 0.125
or 1 : 5, so the formula of the hydrated salt is CuSO4.5H2O
-
The formula mass Mr
can then be calculated as follows:
-
There are some
More
sophisticated A Level problems involving titrations to determine the water of
crystallisation in a salt like compound (See Q30)
See also
14.1
% purity of a product and assay calculations
14.2a
% reaction yield and theoretical yield calculations
and why you can't actually get 100% yield in practice
14.2b atom economy calculations
*
14.3 dilution of solutions calculations
14.5
how
much of a reactant is needed? calculation of quantities required, limiting
reactant quantities
Chemical &
Pharmaceutical Industry Economics & Sustainability, Life Cycle
Assessment, Recycling
TOP OF PAGE

Above is typical periodic table used in GCSE science-chemistry specifications in
doing chemical calculations,
and I've 'usually' used these values in my exemplar calculations to cover most
syllabuses
OTHER CALCULATION PAGES
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What is relative atomic mass?,
relative isotopic mass and calculating relative atomic mass
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Calculating relative
formula/molecular mass of a compound or element molecule
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Law of Conservation of Mass and simple reacting mass calculations
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Composition by percentage mass of elements
in a compound
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Empirical formula and formula mass of a compound from reacting masses
(easy start, not using moles)
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Reacting mass ratio calculations of reactants and products
from equations
(NOT using
moles) and brief mention of actual percent % yield and theoretical yield,
atom economy
and formula mass determination
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Introducing moles: The connection between moles, mass and formula mass - the basis of reacting mole ratio calculations
(relating reacting masses and formula
mass)
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Using
moles to calculate empirical formula and deduce molecular formula of a compound/molecule
(starting with reacting masses or % composition)
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Moles and the molar volume of a gas, Avogadro's Law
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Reacting gas volume
ratios, Avogadro's Law
and Gay-Lussac's Law (ratio of gaseous
reactants-products)
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Molarity, volumes and solution
concentrations (and diagrams of apparatus)
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How to do acid-alkali
titration calculations, diagrams of apparatus, details of procedures
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Electrolysis products calculations (negative cathode and positive anode products)
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Other calculations
e.g. % purity, % percentage & theoretical yield, dilution of solutions
(and diagrams of apparatus), water of crystallisation, quantity of reactants
required, atom economy
-
Energy transfers in physical/chemical changes,
exothermic/endothermic reactions
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Gas calculations involving PVT relationships,
Boyle's and Charles Laws
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Radioactivity & half-life calculations including
dating materials
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Some Advanced A Level Practical Exercises and Calculations
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