Water of crystallization is the fixed number of water molecules present in the crystal of that salt. The chemical formula of hydrated copper sulphate is CuSO4.5H2O i.e., there are five water molecules present in one formula unit of copper sulphate. Therefore, water of crystallization of copper sulphate is 5.

Similarly, there are two water molecules present in one formula unit of gypsum (CaSO4.2H2O). Hence, water of crystallization of gypsum is 2.

however in hemihydrated Caso4 is known as POP it has half water of crystalization. Pl explain

⇒Water of crystallization is the fixed number of water molecules present in the crystal of that salt. The chemical formula of hydrated copper sulphate is CuSO4.5H2O indicates 1 units of copper sulfate share five molecule of water which on heating gives :

CuSO4•5H2O (s) → CuSO4 (s) + 5 H2O (g)

⇒Chemical formula of of gypsum (CaSO4.2H2O) which indicates that 1 units of calcium sulfate share two molecule of water on heating :

CaSO4.2H2O  (s) → CaSO4 (s) + 2H2O (g)

⇒Chemical formula of POP is CaSO4.1/2H2O. This chemical formula tells us that 2 units of calcium sulfate share one molecule  of water.(i.e. if we multiply compound by factor 2 then its will be 2CaSO4.1H2O which is equivalent to CaSO4.1/2H2O ) on heating gives

CaSO4.1/2H2O  (s) → CaSO4 (s) + 1/2H2O (g)

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