Water of Crystallization
• LQ: How can the water of crystallization
be calculated?
Outcomes:
• Understand what ‘water of crystallization’
is and how it is represented.
• Know how to calculate water of
crystallization from experimental data.
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Hydration of anhydrous Copper Sulphate
• Hydration means addition
of water.
• Dehydration means
removing water.
• Copper sulphate is a blue
crystal where 5 water
molecules are bonded to
the copper sulphate
crystal.
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What are the rules for calculating
empirical formula from masses?
 Step 1.
Write down the formula of the compound and its
water of crystallization .
 Step 2.
Write down the masses.
 Step 3.
Write down the relative formula
masses of the elements.
 Step 4.
Divide the masses by their relative formula mass.
 Step 5.
Simplify the ratio by dividing it by the
smallest number.
 Step 6.
Use the molar ratio obtained to determine
empirical formula.
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Determine the value of x in washing soda
crystals Na2CO3.xH2O. Mass of Na2CO3.xH2O =
28.6g before heating. Mass after heating = 10.6
g. Mass of water lost = 18.0 g
Names
Masses M
Na2CO3
10.6 g
H2O
18.0 g
Relative formula mass Mr
Divide M/Mr (No. of moles)
Divide by smallest number
106
0.1
0.1/0.1
18
1.0
1.0/0.1
Ratio of moles of each
element
Empirical formula
1
10
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Na2CO3.10 H2O
© Boardworks Ltd 2007
Determine the value of x in washing soda
crystals Na2CO3.xH2O. Mass of Na2CO3.xH2O =
28.6g before heating. Mass after heating = 10.6
g. Mass of water lost = 18.0 g
Names
Masses M
Na2CO3
H2O
Relative formula mass Mr
Divide M/Mr (No. of moles)
Divide by smallest number
Ratio of moles of each
element
Empirical formula
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A sample of hydrated copper sulphate was with
a mass of 0.869g was heated to drive off the
water of crystallization. The mass was then
found to be 0.556g. Find x in CuSO4.xH2O
Names
Masses M
Relative formula mass Mr
Divide M/Mr (No. of moles)
Divide by smallest number
Ratio of moles of each
element
Empirical formula
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A sample of hydrated calcium sulfate was with a
mass of 16.4g was heated to drive off the water
of crystallization. The mass was then found to
be 3.4 g. Find x in CaSO4.xH2O
Names
Masses M
Relative formula mass Mr
Divide M/Mr (No. of moles)
Divide by smallest number
Ratio of moles of each
element
Empirical formula
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A hydrate is determined to be 45.43% water and
54.57% CoCl2. Find the chemical formula of this
hydrate.
Hint: assume that is 100 g total of the hydrate compound
Names
Masses M
Relative formula mass Mr
Divide M/Mr (No. of moles)
Divide by smallest number
Ratio of moles of each
element
Empirical formula
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Heating hydrated Barium chloride BaCl2.xH2O, in a crucible forms
anhydrous Barium chloride, BaCl2 . Use the experimental data
below to find the value of X and write the formula of the hydrated
salt.
Mass of empty crucible= 30.000 g,
Mass of crucible + BaCl2.xH2O = 32.44 g
Mass of crucible + BaCl2 = 32.08 g
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13.2 g of a sample of zinc sulphate, ZnSO4.xH2O, was strongly
heated until no further change in mass was recorded. On
heating, all the water of crystallisation evaporated as follows:
ZnSO4.xH2O  ZnSO4 + xH2O.
Calculate the number of moles of water of
crystallisation in the zinc sulphate sample given that 7.4 g of
solid remained after strong heating.
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