5. Water of crystallisation
in salt crystals
The explanation and calculation of water of
crystallisation
(Suitable for AQA, Edexcel and OCR GCSE
chemistry students)
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5.
Water of crystallisation calculations
-
What is water of crystallisation?
-
Water of crystallization
are the
molecules of water that are incorporated into some salt crystals when they
are crystallised out of water.
-
e.g. when blue crystals of copper(II) sulfate
are crystallized out of water the actual formula of the crystals is ...
-
NOT
simply CuSO4, but on crystallisation CuSO4.5H2O is formed,
-
because five water molecules
are associated with each 'CuSO4'. in its 'hydrated' crystalline
form.
-
Solubility graphs and data
are covered in section 4.
-
How to calculate the
theoretical % of water in a hydrated salt
-
eg magnesium sulphate MgSO4.7H2O
salt crystals
-
Relative atomic masses:
Mg = 24, S = 32, O = 16 and H = 1
-
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
-
so % water = 126
x 100 / 246 = 51.2%
-
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 mass of the anhydrous salt left
is measured.
The original mass of hydrated salt and the mass of the anhydrous salt
residue can be worked out from the various weighings.
-
The % water of
crystallisation and the formula 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 4.00g. 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's
Cu=64, S=32, O=16, H=1 ]
-
The mass ratio of CuSO4
: H2O is 4.00 : 2.25
-
To convert from mass
ratio to mole ratio, you divide by the molecular mass of each
'species'
-
CuSO4 = 64
+ 32 + (4x18) = 160 and 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
-
All concentration calculations are covered on the
on-line
calculations page, especially sections 7. on molarity, 11. and 12. on molarity and
acid-base (alkali) titrations, section 14.3 on dilutions.
-
GCE A level advanced notes on the
structure of hydrated salts
Extra Aqueous Chemistry
Index:
1. Water
cycle, treatment, pollution
2.
Colloids - sols, foam and emulsions
3.
Hard
and soft water - causes and treatment
4.
Gas and salt solubility
in water and solubility curves
5.
Calculation of water of crystallisation (this page)
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