Formula Of A Hydrated Salt

Determination of formula of hydrated magnesium sulfate.

Definitions:

• Water of crystallisation refers to water molecules that form an essential part of the crystalline structure of a compound.

• Hydrated refers to a crystalline compound containing water molecules.

• Anhydrous refers to a substance that contains no water molecules.

In the molecular formula, the salt and the water are separated by a dot.

A common simple experiment is to determine the formula of a hydrated ionic compound (that does not decompose when heated). In this case is is MgSO₄.xH₂O. x is the unknown here and must be found experimentally, using moles calculations.


Magnesium sulfate crystals are heated in a crucible to dryness using a busnsen flame. A crucible lid prevents any `spitting out` of material during the removal of water. Steps taken are as follows:

1. Weigh crucible.

2. Weigh crucible + hydrated magnesium sulfate.

3. Weigh crucible and anhydrous magnesium sulfate (when cool!).

From an experiment to determine the formula of hydrated magnesiumsulfate:
Mass of hydrated salt MgSO₄·xH₂O = 4.312g
Mass of anhydrous salt MgSO₄ = 2.107g
Mass of H₂O in hydrated salt =
Calculate the amount, in mol, of anhydrous MgSO₄
Calculate the amount, in mol, of water.

Determine the formula of the hydrated salt by working out the molar ratios
Extra: Work out percentage of water by mass in hydrated salt.

After the experiment:
Mass of hydrated salt MgSO4·xH₂O = 4.312g
Mass of anhydrous salt MgSO₄ = 2.107g
Mass of H₂O in hydrated salt = 2.205g
Formula mass of MgSO₄ = 120.4 gmol^-1 (calculated from exam board data booklet)
Formula mass of H₂O = 18.0 gmol^-1
Moles of anhydrous MgSO₄ = 2.107/120.4 = 0.0175 moles
Moles of H₂O = 2.205/18.0 = 0.1225 moles

x = 0.1225 / 0.0175 = 7 so MgSO₄.7H₂O

Formula mass of MgSO₄.7H₂O = 246.4 gmol^-1.
% water in crystals = (7 x H₂O) / MgSO₄.7H₂O = (126 / 246.4) x 100% = 51.1%

Can you think of another ionic compound where this technique may be used?
Copper(II) sulfate (CuSO₄.5H₂O) in this specific case, colour changes from blue to colourless.

Alternatively, for some compounds used in acid-base titrations, x may be also determined from such experiments using volumetric analysis (e.g. crystalline oxalic acid dissolved in water).