Consider the following compounds : $\mathrm{FeSO_4}$ ; $\mathrm{CuSO_4}$ ; $\mathrm{CaSO_4}$ ; $\mathrm{Na_2CO_3}$ The compound having maximum number of water of crystallisation in its crystalline form in one molecule is :

Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. Based on the chemical formulas of the crystalline forms of these compounds:

1. Ferrous sulphate: As mentioned in Activity 1.5 of the text, its crystalline form is $\mathrm{FeSO_4 \cdot 7H_2O}$, containing 7 molecules of water.
2. Copper sulphate: Its crystalline form is $\mathrm{CuSO_4 \cdot 5H_2O}$, containing 5 molecules of water.
3. Calcium sulphate: Its crystalline form (Gypsum) is $\mathrm{CaSO_4 \cdot 2H_2O}$, containing 2 molecules of water.
4. Sodium carbonate: Its crystalline form (Washing soda) is $\mathrm{Na_2CO_3 \cdot 10H_2O}$, containing 10 molecules of water.

Comparing these, $\mathrm{Na_2CO_3}$ has the maximum number (10) of water molecules per formula unit.