1. Water of crystallisation

2. Water of crystallisation
1. Explain the terms anhydrous, hydrated and water of crystallisation
2. Calculate the formula of a hydrated salt using percentage composition, mass composition or experimental data.
Keywords
hydrated
anhydrous
dot formula

3. Hydrated copper (II) sulphate
Anhydrous copper (II)
sulphate
Hydrated copper (II) sulphate
CuSO4 . 5H2O
CuSO4
______________
______________

4. Water molecules are trapped within the crystal structure
This is called water of crystallisation
CoCl2.6H2O

5. CuSO4 5H2O CoCl2 6H2O Na2SO4 10H2O copper sulphate pentahydrate
cobalt chloride hexahydrate
Na2SO4 10H2O
sodium sulphate decahydrate

6. What happens if we don’t know the dot formula?
Rule: use the hydrogens to work the dot formula out NOT the oxygens
Salt
Empirical Formula
Dot Formula
Hydrated magnesium chloride
MgCl2H10O5
Hydrated sodium carbonate
Na2CH20O13
Hydrated calcium nitrate
CaN2H8O10
MgCl2 5H2O
Na2CO3 10H2O
Ca(NO3)2 4H2O

7. What happens if you’re told nothing but still have to work out the water of crystallisation?!
Find the n in BaCl2 . nH2O

8. You have to do it experimentally:
Use heat to drive off the water! This makes the BaCl2 anhydrous.
Mass of crucible: g
Mass of crucible + BaCl2.nH2O crystals: g
Mass of crucible + anhydrous BaCl2: g
Mass of BaCl2 : 2.08 g
Mass of H2O : 0.36 g

9. You have to do it experimentally:
Use heat to drive off the water! This makes the BaCl2 anhydrous.
Mass of crucible: g
Mass of crucible + BaCl2.nH2O crystals: g
Mass of crucible + anhydrous BaCl2: g
Mass of BaCl2 : 2.08 g
Mass of H2O : 0.36 g

10. 3. Number of moles (mass / M) 4. Ratio 5. Empirical formula
(RAMs: H=1, O=16, Cl = 35.5, Ba = 137 )
Substances
Anhydrous BaCl2
H2O
1. Masses (g)
3. Number of moles (mass / M)
4. Ratio
5. Empirical formula
2.08
0.36
2.08/208 = 0.01
0.36/18 = 0.02
Divide by smallest
1:2
BaCl2 . 2 H2O

11. Water of Crystallisation
Questions:
You are supplied with three empirical formulae. Write down the dot formula of each salt to show the water of crystallisation.
a) BaCl2H4O2 b)ZnSH14O11 c) FeN3H12O15
2) From the data below work out the formula of the hydrated salt.
Mass of CaCl2.xH2O = g
Mass of CaCl = g

12. CaCl2.nH2O: 6.573 g anhydrous CaCl2: 3.333 g Mass of H2O : 3.24 g
(RAMs: H=1, O=16, 40 = 35.5, Cl = 35.5 )
Substances
Anhydrous CaCl2
H2O
1. Masses (g)
3. Number of moles (mass / M)
4. Ratio
5. Empirical formula
3.333
3.24
3.333/111 = 0.03
3.24/18 = 0.18
Divide by smallest
1:6
CaCl2 . 6 H2O

13. Example questions
A sample of hydrated magnesium sulfate was strongly heated. The mass of magnesium sulfate before and after heating were recorded in the table below.
Calculate the formula of hydrated magnesium sulfate
Mass of samples before heating (g)
1.232
Mass of sample after heating (g)
0.602
Mass of water lost during heating (g)

14. Example questions
A sample of hydrated zinc sulfate was strongly heated. The mass of zinc sulfate before and after heating were recorded in the table below.
Calculate the formula of hydrated zinc sulfate
Mass of samples before heating (g)
2.875
Mass of sample after heating (g)
1.615
Mass of water lost during heating (g)

15. Example questions
A sample of hydrated magnesium chloride was strongly heated. The mass of magnesium chloride before and after heating were recorded in the table below.
Calculate the formula of hydrated magnesium chloride
Mass of samples before heating (g)
3.706
Mass of sample after heating (g)
1.906
Mass of water lost during heating (g)

16. Plenary
Complete the exam style questions on water of crystallisation, ionic and covalent bonding