Question

Use calculations to show what is the effect on the value of the water of hydration should you fail to perform ignition on your crucible. Explain your answer.

Answer 1

Hydrates are crystalline salts that are bonded to water molecules in definite proportions. The weakly bound water is known as either the water of hydration or water of crystallization.

The fixed numbers of water molecules that are weakly bonded to the salt are represented as follows: salt • number of waters magnesium sulfate heptahydrate (MgSO4 • 7 H2O) The dot represents the weak salt • water bond in the chemical formula.

The bond is so weak that simply heating the hydrated salt to liberate the water molecules as vapour can normally break it.

If water is added to the now anhydrous salt, the reverse will take place with the waters reattaching themselves to the salt. This is known as a reversible action.

(hydrate) (anhydrate) (water)

MgSO4 • 7 H2O ←⎯→ MgSO4 + 7 H2O

As previously mentioned, each hydrated salt has water molecules bonded to them in definite proportions. The percent water in the hydrated salt can be calculated, theoretically, using the chemical formula of the hydrate.

%H2O = 100 * [(#H2O molecules)*(molecular mass H2O)] / (molecular mass of hydrate)

If the crucible was not ignited the heating could not separate the H2O molecules and the results will be all wrong, let's see an example:

Calcium chloride hexahydrate has the chemical formula CaCl2 • 6 H2O. The theoretical percentage of water will be:

The formula weight of CaCl2 • 6 H2O is:

Ca: 1 x 40.1 = 40.1 amu

Cl: 2 x 35.5 = 71.0 amu

H: 12 x 1.0 = 12.0 amu

O: 6 x 16.0 = 96.0 amu

Formula weight of CaCl2 • 6 H2O = 219.1 amu

Formula weight of H2O = 18 amu

%H2O = 100 * [(6)*(18amu)] / (219.1amu)

%H2O = 49.30 %

If accepted value of the compound 35.00% water.

% Error = 100 * (49.30 - 45.00)/(45.00)

% Error = 9.56 %

The value of Water of Hydration will be 9.56 % more heavier in the real compound, so it could fail to perform ignition if you don't heat a little be the hydrate.