7 PAG1

The water of crystallisation in calcium sulphate crystals can be removed as water vapour by heating as shown in the following equation. CaSO4 .xH2O(s) → CaSO4 (s) + xH2O(g) Method. •Weigh an empty clean dry crucible and lid . •Add 2g of hydrated calcium sulphate to the crucible and weigh again •Heat strongly with a Bunsen for a couple of minutes •Allow to cool •Weigh the crucible and contents again •Heat crucible again and reweigh until you reach a constant mass ( do this to ensure reaction is complete). Small amounts the solid , such as 0.100 g, should not be used in this experiment as errors in weighing are too high. Large amounts of hydrated calcium sulphate, such as 50g, should not be used in this experiment as the decomposition is like to be incomplete. The lid improves the accuracy of the experiment as it prevents loss of solid from the crucible but should be loose fitting to allow gas to escape. The crucible needs to be dry otherwise a wet crucible would give an inaccurate result. It would cause mass loss to be too large as water would be lost when heating. Heating in a crucible This method could be used for measuring mass loss in various thermal decomposition reactions and also for mass gain when reacting magnesium in oxygen.N Goalby chemrevise.org A Hydrated salt contains water of crystallisation H. Cu(NO3 )2 .6H2O hydrated copper (II) nitrate(V). Cu(NO3 )2 Anhydrous copper (II) nitrate(V). Example 4 Na2SO4 . xH2O has a molar mass of 322.1, Calculate the value of x Molar mass xH2O = 322.1 – (23×2 + 32.1 + 16×4) = 180 X = 180/18 =10