Hydrated Salts | AQA AS Chemistry Revision Notes 2016

Water of Crystallisation

Water of crystallisation is when some compounds can form crystals which have water as part of their structure
A compound that contains water of crystallisation is called a hydrated compound
The water of crystallisation is separated from the main formula by a dot when writing the chemical formula of hydrated compounds
- E.g. hydrated copper(II) sulfate is CuSO₄∙5H₂O
A compound which doesn’t contain water of crystallisation is called an anhydrous compound
- E.g. anhydrous copper(II) sulfate is CuSO₄
A compound can be hydrated to different degrees
- E.g. cobalt(II) chloride can be hydrated by six or two water molecules
- CoCl₂ ∙6H₂O or CoCl₂ ∙2H₂O

The conversion of anhydrous compounds to hydrated compounds is reversible by heating the hydrated salt:

Hydrated: CuSO₄•5H₂O ⇌ CuSO₄ + 5H₂O :Anhydrous

The degree of hydration can be calculated from experimental results:
- The mass of the hydrated salt must be measured before heating
- The salt is then heated until it reaches a constant mass
- The two mass values can be used to calculate the number of moles of water in the hydrated salt - known as the water of crystallisation

Worked example

Calculating water of crystallisation

11.25 g of hydrated copper sulfate, CuSO₄.xH₂O, is heated until a constant mass of 7.19 g.

Calculate the formula of the hydrated copper(II) sulfate.

A_r (Cu) = 63.5 A_r (S) = 3. A_r (O) = 16 A_r (H) = 1

Answer:

1. Salt and water	CuSO₄	H₂O
2. Value	7.19	11.25 - 7.19 = 4.06
3. M_r	63.5 + 32 + (16 x 4) = 159.5	(1 x 2) + 16 = 18
4. Moles = $\frac{mass}{M_{r}}$	$\frac{7.19}{159.5}$ = 0.045	$\frac{4.06}{18}$ = 0.226
5. Salt : water ratio	$\frac{0.045}{0.045}$ = 1	$\frac{0.226}{0.045}$ = 5
6. Formula of hydrated salt	The formula is CuSO₄•5H₂O

Examiner Tip

A water of crystallisation calculation can be completed in a similar fashion to an empirical formula calculation

Instead of elements, you start with the salt and water
Instead of dividing by atomic masses, you divide by molecular / formula masses
The rest of the calculation works the same way as the empirical formula calculation

Hydrated Salts (AQA AS Chemistry)

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1. Physical Chemistry

1.1 Atomic Structure

1.1.1 Fundamental Particles

1.1.2 Mass Number & Isotopes

1.1.3 Time of Flight Mass Spectrometry

1.1.4 Shells & Orbitals

1.1.5 Electron Configuration

1.1.6 Ionisation Energy

1.1.7 Ionisation Energy: Trends & Evidence

1.2 Formulae, Equations & Calculations

1.2.1 Relative Atomic Mass & Relative Molecular Mass

1.2.2 Empirical & Molecular Formula

1.2.3 Balanced Equations

1.2.4 Reaction Yields

1.2.5 Atom Economy

1.2.6 Hydrated Salts

1.3 The Mole, Avogadro & The Ideal Gas Equation

1.3.1 The Mole & the Avogadro Constant

1.3.2 Reacting Masses

1.3.3 Reacting Volumes

1.3.4 The Ideal Gas Equation

1.4 Types of Bonding & Properties

1.4.1 Changes in State: Energy Changes

1.4.2 Ionic Bonding

1.4.3 Covalent Bonding

1.4.4 Dative Covalent Bonding

1.4.5 Dot & Cross Diagrams

1.4.6 Metallic Bonding

1.4.7 Properties of Ionic Compounds

1.4.8 Properties of Covalent Substances

1.4.9 Properties of Metallic Substances

1.4.10 Effects of Structure & Bonding

1.5 Molecules: Shapes & Forces

1.5.1 Shapes of Simple Molecules & Ions

1.5.2 Bond Polarity

1.5.3 Types of Forces Between Molecules

1.5.4 Effects of Forces Between Molecules

1.6 Energetics

1.6.1 Bond Energy

1.6.2 Energy Level Diagrams

1.6.3 Enthalpy Changes

1.6.4 Calorimetry

1.6.5 Hess' Law

1.6.6 Applications of Hess’s Law

1.6.7 Bond Enthalpies

1.7 Kinetics

1.7.1 Collision Theory

1.7.2 Measuring Rates of Reaction

1.7.3 Maxwell–Boltzmann Distributions

1.7.4 Effect of Temperature on Reaction Rate

1.7.5 Effect of Concentration & Pressure

1.7.6 Catalysts

1.8 Chemical Equilibria, Le Chatelier's Principle & Kc

1.8.1 Chemical Equilibria

1.8.2 Le Chatelier's principle

1.8.3 The Equilibrium Constant, Kc

1.8.4 Calculations Involving the Equilibrium Constant

1.8.5 Changes Which Affect the Equilibrium

1.9 Oxidation, Reduction & Redox Equations

1.9.1 Oxidation & Reduction

1.9.2 Oxidation States: The Rules

1.9.3 Redox Equations

2. Inorganic Chemistry

2.1 Periodicity

2.1.1 Classification of an Element

2.1.2 Trends of Period 3 Elements: Atomic Radius

2.1.3 Trends of Period 3 Elements: First Ionisation Energy

2.1.4 Trends of Period 3 Elements: Melting Point

2.2 Group 2, the Alkaline Earth Metals

2.2.1 Trends in Group 2: The Alkaline Earth Metals

2.2.2 Solubility of Group 2 Compounds: Hydroxides & Sulfates

2.2.3 Reactions of Group 2

2.2.4 Uses of Group 2 Elements

2.3 Group 7 (17), the Halogens

2.3.1 Physical Properties of Group 7