Question

How do you write the formula for the following compounds: Barium chloride dihydrate and Sodium sulfite heptahydrate?

Answer 1

`BaCl_2*2H_2O`, and `Na_2SO_3*7H_2O`

Explanation:

`"Hydrate"` refers to a `"water of crystallization"`. `"Sulfite"` refers to the `SO_3^(2-)` counterion. When these material crystallize from water as their metal salts, the salt picks up several water molecules that are included in the solid state structure, the crystal (hence `"a water of crystallization"`).

We generally write these as shown. If we dried the salt, we could potentially remove the volatile water molecules; this would be a non-trivial exercise, but most of the time, inorganic chemists, who tend to use non-aqueous solvents, would dry the salt before use.