Question

Give the chemical formula of Carnallite.

Answer 1

Hint: Carnallite is a double salt with orthorhombic crystals and is not very common. The chemical formula of Carnallite can be written using the two ionic salts that it is made up of and the number of water molecules present as water of crystallization.

Complete answer:
The chemical formula of any compound or complex is a condensed formula that includes the chemical symbols of all the elements present in the compound. The symbol of each element is subscripted with its stoichiometric number indicating the number of atoms of that element present per molecule of the compound.
The chemical formula gives us additional information about the significance of certain molecules in the compound or complex. If the chemical symbols are written inside square brackets then it indicates a complex. If the formula of two ionic salts is separated by dots then it is called a double salt. For example alum is a double salt made up of sulphates of two different metal ions like potash alum: \[{K_2}S{O_4}.A{l_2}{(S{O_4})_3}.24{H_2}O\]
A double salt is different from a coordination complex. All the constituents of double salt that crystallize together get dissociated in water but the ions present or molecules that are part of the complex remain intact.
Carnallite is a double chloride salt. It contains two ionic salts in one compound. Both potassium and magnesium ions are present as their chlorides and the two salts crystallize together to form an orthorhombic geometry. The chemical formula of Carnallite is written using the same rules as that of all double salts i.e. the formula of the constituent salts is separated by dots.
Thus the formula of Carnallite is \[KCl.MgC{l_2}.6{H_2}O\]

Note:
The six water molecules written separately after a dot in the chemical formula of Carnallite represent the water of hydration or crystallization. This indicates that each crystal contains a total of six water molecules trapped inside the crystal along with the salts present in it.