Water of crystallisation
Lesson objectives
At the of the lesson you should be able….
a. Explain the terms anhydrous, hydrated and water of
crystallisation.
b. calculate the formula of a hydrated salt using percentage
composition, mass composition or experimental data
Anhydrous and hydrated salts
Most salts exist in the solid form either as pure substance or in the form of
crystals that contain water molecules as part of their structure.
An anhydrous salt is refers to a salt that contains no water molecules.
Example: Sodium sulphate
Na2SO4
A hydrated salt refers to a salt(crystalline compound) containing water
molecules.
Examples
CuSO4.5H2O
CoCl2.6H2O
FeSO4.7H2O
Na2SO4.10H2O
Copper sulphate pentahydrate
Cobalt chloride hexahydrate
Iron sulphate heptahydrate
Sodium sulphate decahydrate
EMPIRICAL VS DOT FORMULAE
Salt
Empirical formula
Dot formula
Hydrated copper
sulphate
CuSH10O9
CuSO4.5H2O
Hydrated sodium
carbonate
Na2CH20O13
NaCO3.10H2O
Hydrated calcium
nitrate
CaN2H8O10
Ca(NO3).4H2O
Note: The relative number of water molecule in a hydrated salt is
indicated by a number after the dot. Thus, CuSO4.5H2O has 5 molecules
of water of crystallisation.
Determination of the formula of hydrated salt
The formula of a hydrated salt can be determined by heating so that
the water of crystallisation evaporates completely.
The following are determined from the results:
a. the mass of the hydrated salt, containing water of crystallisation
b. the mass of the anhydrous salt, without the water
c. the mass of water that was in the hydrated salt
From the above you then calculate:
1. The amount, in mol, of water
2. The amount, in mol, of the anhydrous salt
3. The mole ratio of water and the anhydrous salt, followed by the
simplest ratio.
Finally, Write the formula of the hydrated salt by using the ratio.
1. A sample of copper crystals has a mass of 6.80 g. The sample is
heated to drive off all the water of crystallisation. The mass is
reduced to 4.35 g. Calculate the formula of the copper sulphate
crystals.
Answer
mass of hydrated salt = 6.80 g
mass of anhydrous salt = 4.35 g
mass of water driven off by heating = 6.80 – 4.35 = 2.45 g
amount , in mol, of water of crystallisation, n =m = 2.45 = 0.136 mol
M
18
amount, in mol, anhydrous copper sulphate(CuSO4), n = m
=
4.35
= 4.35 = 0.02726 mol
63.5+32.1+(16x4)
159.6
mole ratio
simplest ratio
CuSO4 : H2O = 0.02726 :
M
0.136
0.02726 = 1
0.136 = 5
i.e. 1:5
0.02726
0.02726
Therefore, the formula of the hydrated copper sulphate is CuSO4.5H2O
A sample of magnesium crystals contains 9.78% Mg, 38.69% SO42-, and
51.53% H2O by mass. Determine the formula of the crystals.
( S = 32.1, H = 1.0, O = 16.0, Mg = 24.3 )
ANSWER
Mole ratio
amount, in mol, of magnesium, n = m = 9.78 = 0.4025 mol
M
24.3
amount, in mol, of sulphate, n = m = 38.69 = 0.4026 mol
M
96.1
amount, in mol, of water, n = m = 51.53 = 2.863 mol
M
18
simplest ratio
0.4025 = 1
0.4025
0.4026 = 1
0.4026
2.863 = 7
0.4025
Therefore, the formula of the magnesium sulphate crystals is MgSO4.7H2O
Now try question 1 – 2
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