Water of crystallisation
L.O.:
Explain the terms anhydrous, hydrated and
water of crystallisation.
Calculate the formula of a hydrated salt.
Workshop today at 1:45pm:
Winifred
Henry
Anna
Keverne
Esther Adeduntan
Copper sulfate
• Copper sulfate comes in a hydrated form
(copper sulfate pentahydrate) and a
anhydrous form
http://uk.youtube.com/watch?v=kp7yaxlYk08
Anhydrous substances
• Many substances will absorb water from the
atmosphere, like NaCl, these are called
hygroscopic substances
• An anhydrous compound is one which has
no water molecules inside of it
• To make an anhydrous substance you can
either heat it to evaporate off the water, or
you can dry it in a drying cabinet
Writing out the formula for hydrated
compounds
•
There are two ways in which a formula can
be written if it is a hydrated compound:
1) Empirical formula, Which you know and love
OR
2) Dot formula, So copper sulfate
pentahydrate becomes Cu SO3 • 5H2O
NOTE: This dot is very
important!!!!!
Hydrated substances and water of
crystallisation
• Some substances can trap water molecules
inside of them.
• The water contained is called the water of
crystallisation
Finding the formula of hydrated
copper(II) sulfate
Note: see
worked
example
in page 27
FeSO4·xH2O
Exp 1
Mass of crucible
Mass of crucible
+ hydrated salt
Mass of hydrated
salt
Mass of
anhydrous salt
Mass of water
lost
Exp 2
FeSO4·xH2O
Exp 1
Mass of
anhydrous
salt
Mass of
water lost
Exp 2
Mean mass
FeSO4·xH2O
Mass of anhydrous salt
Mean
mass
1.51 g
Mass of water lost
0.54 g
1) Calculate amount in moles of anhydrous salt
n (FeSO4) = 1.51/ 151.8 = 0.01
2) Calculate the amount in moles of water.
n (H2O) = 0.54/18 = 0.03
3) Calculate molar ratio
FeSO4 : H2O = 0.01: 0.03
4) Divide be the smallest number 0.01: 0.03 = 1:3
Salt
CaCl2•2H2O
ZnCl2•6H20
Ba(OH)2•8H2O
Cu SO3 • 5H2O
YSO3•6H20
Na Z
XSO4
Ions present
Hydrated
salt
Mass of anhydrous
salt
Water lost
XCl•2H2O
5.86 g
9g
XSO4•3H20
40.53 g
16.2
Identity of salt
Other examples of hydrated compounds:
• Cobalt chloride hexahydrate
• Sodium sulfate decahydrate
Write out the formula for these compounds
using the “dot formula” notation
Convert these into the Dot Formula
1.
2.
3.
4.
Calcium chloride dihydrate
Calcium chloride hexahydrate
Zinc chloride hexahydrate
Barium hydroxide octahydrate
Answers
1.
2.
3.
4.
Calcium chloride dihydrate = CaCl2•2H2O
Calcium chloride hexahydrate = CaCl2•6H2O
Zinc chloride hexahydrate = ZnCl2•6H20
Barium hydroxide octahydrate = Ba(OH)2•8H2O
Write these out in Dot formula
1. Hydrated Cobalt chloride CoCl2H12O6
2. Hydrated Calcium carbonate CaCH6O6
3. Hydrated Magnesium sulphate MgSH12O10