State the number of water molecules present in crystals of washing soda and Plaster of Paris. What are these water molecules called?

(N/A) $(i)$ Washing soda contains $10$ molecules of water of crystallisation $(Na_2CO_3 \cdot 10H_2O)$. Plaster of Paris contains $1/2$ molecule of water of crystallisation $(CaSO_4 \cdot 1/2H_2O)$.
$(ii)$ These water molecules are known as water of crystallisation.