Water of Crystallization

Water of crystallization is the fixed amount of water, which is necessary for certain salts to crystallize out from their aqueous solutions. This water makes it possible for them to form crystals, and it is responsible for the shapes of their crystals.

The crystals contain the salts and water combined in definite proportions, and when heated, the water is lost. Example, CuSO₄.5H₂O (blue crystals) - the crystals are formed by the chemical combination of 1 mole of CuSO₄ with 5 moles of H₂O.

When heated, it looses the water of crystallization (i.e. 5 moles) and forms a white powder, CuSO₄. Other examples of salts which crystallize with water are Na₂CO₃.10H₂O (crystallizes with 10 moles of H₂O); Na₂SO₄.10H₂O (with 10 moles of H₂O) and FeSO₄.7H₂O (with 7 moles of H₂O).

From studies, it is found that the water of crystallization actually form chemical bonds with the positive and negative ions of the salts. Example, in CuSO₄.5H₂O, 4 moles of H₂O are in coordinate bonding with Cu²⁺, while 1 mole of water forms hydrogen bonding with the SO₄^2- ion. In some hydrated salts however, some of the water do not get attached by chemical bonds, but occupy certain positions in the crystal structure - such water is called lattice water.

An example is the hydrate KAl(SO₄)₂.12H₂O, in which 6H₂O molecules are lattice water molecules and the other 6 are water of crystallization because they are attached by coordinate bonds to the aluminium ion. Example: If 0.715 g of a hydrated form of sodium trioxocarbonate(IV) exactly reacts with 50 cm³ of 0.10 M hydrochloric acid, determine the number of moles of water of crystallization present in one mole of the hydrated salt. (Na - 23, C = 12, O = 16, H = 1)

Solution: Take note of the following:

I. The mass of the hydrated salt 0.715 g is the sum of the masses of the anhydrous salt and the water of crystallization present.

2. Only the anhydrous salt will react with the hydrochloric acid. The water molecules present as water of crystallization do not get involved in the reaction. Number of moles of HCl involved in the reaction:

M = No. of moles/V(dm³)

No. of moles = 0.10 x 0.05 dm³ = 0.005 mole

Therefore, from the equation of the reaction:

Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)

1 mole of Na₂CO₃ reacted with 2 moles of HCl, therefore, 0.0025 (i.e. 0.005/2) mole of Na₂CO₃ reacted with 0.005 mole of HCl.

Converting 0.0025 mole of Na₂CO₃ to mass in grams, we have:

Mass (g) = No. of moles x molar mass = 0.0025 x 106 = 0.265 g

Hence, mass of water of crystallization = 0.715 - 0.265 = 0.45 g

No. of moles of water of crystallization = 0.45/18 = 0.025 mole

Therefore, the mole ratio between Na₂CO₃ and H₂O = 0.0025 : 0.025

I.e. 0.0025/0.0025 : 0.025/0.0025

1 : 10

Therefore, the molecular formula of the hydrated salt is Na₂CO₃ . 10H₂O

Hence, in one mole of the hydrated salt 10 moles of water of crystallization are contained.

Note: - The combination between the water of crystallization and the salt ions is a chemical reaction - a proof is that certain amount of heat (heat of hydration) is given off when a salt crystallizes from its solution.

Also, a change is observed in both the appearance and the mass of the salt, before and after crystallization (i.e., the anhydrous or powdery salt compared with the crystalline form.

- There is no difference between the solutions obtained by dissolving both anhydrous salt and salt with water of crystallization. Example, the solution of CuSO₄ is same as that of CuSO₄. 5H₂O.