Hydrated Salts

Water of Crystallisation: Water molecules that are bounded into crystalline structure of a
• Dot or .xH2O shows number of waters of crystallisation. Anhydrous means no waters of
• Copper (II) Sulphate: When blue crystals of hydrated copper sulphate are heated, bonds holding
the water within the crystal broken. Crystalline structure is lost. Leaves white anhydrous copper
sulphate. Difficult to remove last traces of water, so can be very pale blue.
• Experiment: Weigh empty crucible. Weigh the crucible and the hydrated salt in it. Weigh crucible
and the anhydrous salt in it.
• When using hydrated salt in experiments, don’t know how much water is in the mass.
• Calculating Amount of H2O:
1- Find moles of anhydrous salt. Need to takeaway values.
2- Find moles of water. Need to takeaway values.
3- Find the simplest molar ratio e.g. 1: 5 by dividing moles of water by moles of salt. 5 would be
the amount of H2O molecules.
4- OR work out by subtracting molar mass of salt from salt + water compound and use remaining
molar mass to work out H2O number in front.
• Find Expected Mass of Anhydrous Salt: Find moles of hydrated salt. Use molar mass to find moles
of anhydrous salt.
• Converting Empirical to Dot Formula: Hydrogen is halved to be the number in front of H2O. Rest
of oxygen with other ion compound.
• Assumptions:
– All Water Lost: Some water may be left inside even if colours different. Difficult if both salt
forms, same colour. Must reheat salt repeatedly so mass remains constant which suggests all
water removed.
– Further Decomposition: Salts decompose further when heated. Copper (II) sulphate heated
very strongly, decomposes to black copper (II) oxide. Not as easy if no colour change.