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