DETERMINATION OF THE WATER OF CRYSTALLISATION
April 4, 2012
Chemical compounds that contain discrete water molecules as part of their crystalline structure are called hydrates. Hydrates occur quite commonly among chemical substances, especially among ionic substances. More often than not, such compounds are either prepared in, or are recrystallized from, aqueous solutions. Hydrates exist for ionic compounds most commonly, but hydrates of polar and non polar covalent molecules are also known. In this experiment, you will study some of the properties and characteristics of several ionic hydrates, and then you will determine the percentage by weight of water in an unknown hydrate as well as its mole ratio of water to anhydrous salt.
CHARACTERISTICS AND PROPERTIES OF THE HYDRATES
Hydrates are most commonly encountered in the study of metal salts, especially the salts of the transition metals. Water is bound in most hydrates in definite, stoichiometric proportions, and the number of water molecules bound per metal ion is often characteristic of a particular metal ion.
A very common hydrate often encountered in the general chemistry laboratory is copper (II) sulfate pentahydrate, CuSO4.5H20. The word “pentahydrate” in the name of this substance indicates that five water molecules are bound in this substance per copper sulfate formula unit. Hydrated water molecules are generally indicated in formulas as shown above for the case of the copper sulfate, using a dot to separate the water molecules from the formula of the salt itself. Although not usually shown as such, you can think of this as CuSO4 (H2O)5.
Many hydrated salts can be transformed to the anhydrous (without water) compound by heat. For example, if a sample of copper sulfate pentahydrate is heated, the bright blue crystals of the hydrate are converted to the white, powdery, anhydrous salt.
CuSO4.5H2O(s) CuSO4(s) + 5H2O(g)
During the... [continues]
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