Water of Crystallisation

Some compounds have water associated with them.

Eg. Cobalt chloride CoCl2 in crystal form is really cobalt chloride CoCl2.6H2O

For each CoCl2 there are 6 H2O molecules in the crystal

When we heat the pale pink crystals to remove the water the crystals lose weight and turn blue

The water that is included in these compounds is called the water of crystallisation, that is the water included in the crystals

Calculating water of crystallisation1. Weigh the hydrous salt2. Dehydrate the salt3. Weigh the anhydrous salt4. Calculate the molar mass of the anhydrous salt5. Calculate the amount of anhydrous salt in moles6. Calculate the mass of the water lost7. Calculate the amount in moles of water lost8. Calculate the ratio of anhydrous salt to water

and convert this to whole numbers9. Write the formula of the hydrated compound

Water of crystallisation calculation

• Find x in the formula MgSO4.xH2O if 5.00g of Magnesium Sulfate was dehydrated and 2.44g remained.