A) Copper sulphate crystals contain five molecules of crystallisation in one formula unit. The chemical formula is CuSO4.5H2O (copper sulphate pentahydrate) .
b) Washing soda crystals (sodium carbonate crystals) contain 10 molecules of crystallisation in one formula unit. The chemical formula is Na2CO3.10H2O (sodium carbonate decahydrate).
c) Gypsum (calcium sulphate crystals) contains two molecules of water of crystallisation in one formula unit. The chemical formula is CaSO4.2H2O (calcium sulphate dihydrate).