The correct answer is option 4, which is 7 water molecules.
Ferrous Sulphate (FeSO4) is a crystal that contains water molecules within its structure. These water molecules are referred to as water of crystallization. The number of water molecules present in a Ferrous Sulphate crystal can be determined by examining its chemical formula.
The chemical formula of Ferrous Sulphate is FeSO4.7H2O. This notation indicates that for each FeSO4 unit, there are 7 water molecules associated with it.
Therefore, the number of water molecules present in a Ferrous Sulphate crystal is 7.
It is important to note that the presence of water molecules in a crystal is common and can significantly impact the properties and behavior of the substance.