Formulae for crystals and salts  

​​In a nutshell 

Salts formed during a reaction can contain water molecules in their lattice structures. This water is known as the water of crystallisation. 

Water of crystallisation 

​​Definition 

The water of crystallisation is known as the the water molecules embedded in the lattice structure of a salt. 

When a salt contains water this salt is known as a hydrated salt. A salt that does not contain water is known as anhydrous. One mole of the hydrated salt is equal to the same number of moles as the water of crystallisation. 

Example 

Hydrated calcium chloride has the formula $$CaCl_2\bullet6H_2O$$.

When hydrated salts are heated, they lose water and become fully anhydrous. 

The formula of a hydrated salt can be found using the following method:

procedure 

Example 

$$21.096 \, g$$ of $$CaSO_4\bullet XH_2O$$ is heated to form $$10.242 \, g$$ of anhydrous calcium sulfate. What is the formula for the hydrated calcium sulfate?​

Find the mass of the water that is lost:  

$$m \, = \, 21.096 \, - \, 10.242 \, = \, 10.854 \, g$$​​

Use the following equation to find the number of moles of the water:

$$n \, = \frac{m}{M}$$​​

Find the molar mass of water ($$H_2O$$):

​$$M \, (H_2O) \, = \, (1 \, \times \, 2) \, + \, 16 \, = \, 18 \, g \, mol^{-1}$$​​

Substitute the values for the mass and molar mass to find the number of moles of the water that is lost:

$$n \, = \, \frac{10.854}{18} \, = \, 0.603 \, mol$$​​

Use the following equation to find the number of moles of the anhydrous salt: 

$$n \, = \frac{m}{M}$$​​

Find the molar mass of the anhydrous salt ($$CaSO_4$$):

​$$M \, (CaSO_4) \, = \, 40.1 \, + \, 32.1 \, + \, (16 \, \times \, 4) \, = \, 136.2 \, g \, mol^{-1}$$​​

Substitute the values for the mass and molar mass to find the number of moles of anhydrous salt:

$$n \, = \, \frac{10.242}{136.2} \, = \, 0.0752 \, mol$$​​

Finally, divide the number of moles of water by the number of moles of the anhydrous salt:

$$\frac{0.603}{0.0752}\, = \, 8.019 \, mol$$

Therefore, the formula for the hydrated calcium sulfate is $$\underline{CaSO_4\bullet 8H_2O}$$.