Water of crystallisation

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. Keywordshydrated

anhydrous

dot formula

Anhydrous copper (II)

sulphate

Hydrated copper (II) sulphate

______________ ______________CuSO4 CuSO4 . 5H2O

Water molecules are trapped within the crystal structure

This is called water of crystallisation

CoCl2.6H2O

CuSO4

5H2O

CoCl2 6H2O

Na2SO4 10H2O

copper sulphate pentahydratecobalt chloride hexahydratesodium sulphate decahydrate

Salt Empirical Formula

Dot Formula

Hydrated magnesium chloride

MgCl2H10O

5

Hydrated sodium carbonate

Na2CH20O1

3

Hydrated calcium nitrate

CaN2H8O10

Rule: use the hydrogens to work the dot formula out NOT the oxygens

What happens if we don’t know the dot formula?

MgCl2 5H2O

Na2CO3 10H2O

Ca(NO3)2 4H2O

What happens if you’re told nothing but still have to work out the water of crystallisation?!

Find the n in BaCl2 . nH2O

You have to do it experimentally:

Mass of crucible: 30.00 gMass of crucible + BaCl2.nH2O crystals: 32.44 gMass of crucible + anhydrous BaCl2: 32.08 g

Use heat to drive off the water! This makes the BaCl2 anhydrous.

Mass of BaCl2 : 2.08 gMass of H2O : 0.36 g

You have to do it experimentally:

Mass of crucible: 30.00 gMass of crucible + BaCl2.nH2O crystals: 32.44 gMass of crucible + anhydrous BaCl2: 32.08 g

Use heat to drive off the water! This makes the BaCl2 anhydrous.

Mass of BaCl2 : 2.08 gMass of H2O : 0.36 g

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

1:2

BaCl2 . 2 H2O

(RAMs: H=1, O=16, Cl = 35.5, Ba = 137 )

Divide by smallest

Water of Crystallisation

Questions:

1) 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 = 6.573 g Mass of CaCl2 = 3.333 g

CaCl2.nH2O: 6.573 ganhydrous CaCl2: 3.333 g

Mass of H2O : 3.24 g

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

1:6

CaCl2 . 6 H2O

(RAMs: H=1, O=16, 40 = 35.5, Cl = 35.5 )

Divide by smallest

Example questionsA 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)

Example questionsA 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)

Example questionsA 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)

Plenary Complete the exam style questions on water

of crystallisation, ionic and covalent bonding