Question

Write the name and chemical formula of two such compounds whose one formula unit is associated with 10 and 2 water molecules respectively.

Answer 1

Washing soda:

1. The chemical name of washing soda is Sodium carbonate decahydrate $({\mathrm{Na}}_2{\mathrm{CO}}_3·10{\mathrm{H}}_2\mathrm{O})$.
2. It has ten water molecules associated with it, hence it is decahydrate.
3. The anhydrous form of washing soda is soda ash i.e. ${\mathrm{Na}}_2{\mathrm{CO}}_3$
4. Therefore, the chemical formula which is having 10 water molecules associated with it is $({\mathrm{Na}}_2{\mathrm{CO}}_3·10{\mathrm{H}}_2\mathrm{O})$.

Gypsum:

1. Gypsum $\mathbf{(}{\mathbf{CaSO}}_{\mathbf{4}}\mathbf{.}\mathbf{2}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{)}$ has 2 water molecules associated with it.
2. When gypsum is heated, it changes to the Plaster of Paris$\left({\mathrm{CaSO}}_4·\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\right)$ and when the Plaster of Paris is heated it gives back Gypsum again.
3. Therefore, the chemical formula which is having 2 water molecules associated with it is $({\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O})$

Hence, the name of two compounds whose one formula unit is associated with 10 and 2 water molecules are Washing soda and Gypsum respectively.