Question

How many molecules of water crystallization (per formula unit) are present in:
a.Copper sulphate crystal
b.Washing soda
c.Gypsum

Answer 1

Water of crystallization or water of hydration are water molecules that are present inside crystals. Water is often incorporated in the formation of crystals from aqueous solutions. Find out the molecular formula of the above compounds. This will directly give the number of water of crystallization

Complete answer:
The water of crystallization means having a fixed number of molecules present in one formula of a unit of salt. We can also say that it is stoichiometrically bound into crystals. Crystal salts with water of crystallization are known as hydrates. The other names of water crystallization are crystallization water or water of hydration.
The water of crystallization is caused by forming purified crystals from an aqueous solution. These crystals do not include contaminants. These crystals are easily affected by heat.
For finding out the water of crystallization we must know the formula of the compounds. In the question we have given three compounds copper sulphate crystal, washing soda and gypsum. The formula for them is as follows:
Copper sulphate crystals - $CuS{O_{4}}.5{H_2}O$
Washing soda- $N{a_2}C{O_3}.10{H_2}O$
Gypsum - $CaS{O_4}.2{H_2}O$
Now by looking at the formulas we can easily tell the number of molecules of water of crystallization.
a.In copper sulphate crystals - $CuS{O_{4}}.5{H_2}O$ there are five molecules of water of crystallization.
b.In Washing soda- $N{a_2}C{O_3}.10{H_2}O$ there are ten molecules of water of crystallization.
c.In Gypsum - $CaS{O_4}.2{H_2}O$there are two molecules of water of crystallization.

Note:
Only hydrated salts contain water of crystallization. Anhydrous salts do not contain water of crystallization. The number of moles of water of crystallization in a compound is indicated by the chemical formula. The number of crystallization water of crystallization in a compound is fixed. The water of crystallization can generally be removed by heating a sample but the crystalline properties are often lost.