SCH 3U - Nomenclature - Hydrates

Usually when an aqueous solution of a salt is allowed to evaporate, all of the water disappears and crystals of the solute are left.  However, there is a group of substances, known as hydrates, which behave differently.  When an aqueous solution of a hydrate evaporates, some of the water remains behind as a part of the crystal structure of the substance.  This water is referred to as the water of hydration or water of crystallization.  The number of molecules of water of crystallization associated with a formula unit of a particular hydrate is always the same and can only be determined by experiment.  The compound without its water of crystallization is known as the anhydrous compound.

Name	Formula
barium hydroxide octahydrate	Ba(OH)2·8H2O
cobalt (II) chloride hexahydrate	CoCl2·6H2O

In the name of the hydrates, the ‘·’ means ‘associated with’.  Prefixes are used to indicate the number of water molecules associated with the salt.  For instance, octahydrate means that the salt has eight water molecules for every one molecule of salt (and this is written ‘·8H₂O’ in the formula).  

Homework

Give the correct formula for the following.

1.     magnesium sulfate heptahydrate

2.    sodium carbonate decahydrate

3.    barium chloride hexahydrate

4.    copper (II) chloride dihydrate

Give the correct name for the following.

1.    CuSO₄·5H₂O

2.    CoCl₂·6H₂O

3.    NiCl₂·6H₂O

4.    BaCl₂·2H₂O

5.    AlCl₃·6H₂O

 

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