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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:
WinifredHenryAnnaKeverneEsther 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 Exp 2Mass of crucibleMass of crucible + hydrated saltMass of hydrated saltMass of anhydrous saltMass of water lost
FeSO4·xH2O
Exp 1 Exp 2 Mean massMass of anhydrous saltMass of water lost
FeSO4·xH2OMean mass
Mass of anhydrous salt 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 = 0.033) Calculate molar ratioFeSO4 : H2O = 0.01: 0.034) Divide be the smallest number 0.01: 0.03 = 1:3
Salt Ions presentCaCl2•2H2O
ZnCl2•6H20
Ba(OH)2•8H2O
Cu SO3 • 5H2O
YSO3•6H20
Na ZXSO4
Hydrated salt
Mass of anhydrous salt
Water lost Identity of salt
XCl•2H2O 5.86 g 9 g
XSO4•3H20 40.53 g 16.2
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. Calcium chloride dihydrate 2. Calcium chloride hexahydrate 3. Zinc chloride hexahydrate4. Barium hydroxide octahydrate
Answers
1. Calcium chloride dihydrate = CaCl2•2H2O
2. Calcium chloride hexahydrate = CaCl2•6H2O
3. Zinc chloride hexahydrate = ZnCl2•6H20
4. 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
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