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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 SO42- 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.


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