Question

Common alum is:
A.${{\text{K}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}{\text{.A}}{{\text{l}}_{\text{2}}}{{\text{(S}}{{\text{O}}_{\text{4}}}{\text{)}}_{\text{3}}}{\text{.24}}{{\text{H}}_{\text{2}}}{\text{O}}$
B.${{\text{(N}}{{\text{H}}_{\text{4}}}{\text{)}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}{\text{.FeS}}{{\text{O}}_{\text{4}}}{\text{.6}}{{\text{H}}_{\text{2}}}{\text{O}}$
C.${{\text{K}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}{\text{.C}}{{\text{r}}_{\text{2}}}{{\text{(S}}{{\text{O}}_{\text{4}}}{\text{)}}_{\text{3}}}{\text{.24}}{{\text{H}}_{\text{2}}}{\text{O}}$
D.${{\text{K}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}{\text{.F}}{{\text{e}}_{\text{2}}}{{\text{(S}}{{\text{O}}_{\text{4}}}{\text{)}}_{\text{3}}}{\text{.24}}{{\text{H}}_{\text{2}}}{\text{O}}$

Answer 1

Hint: Alums are basically aluminium double hydrated salts. The naming of alums is done on the basis of monovalent cations present in the chemical formula of that alum but simply the word ‘alum’ refers to potassium alum.


Complete step by step answer:
Alum is a chemical compound. Basically it is a hydrated double salt of aluminium. The general formula of alum is ${\text{X}}{\text{.Al(S}}{{\text{O}}_{\text{4}}}{{\text{)}}_2}{\text{.12}}{{\text{H}}_{\text{2}}}{\text{O}}$ ,here the $X$ is monovalent cation.
In the place of ${\text{X}}$if potassium is used as a monovalent cation then it is called simply alum. But the naming for other monovalent cations is different. The other alums are named after the cations used such as sodium alum, ammonium alum, etc.
All the aluminium based alums are named after the cations and some of the important alums are following.
Potassium alum, Sodium alum and Ammonium alum.
All the aluminium based alums have common properties in them. Some of the common properties are like, soluble in water, sweet in taste and are acidic in nature. Alums are surrounded by six water molecules so we have to heat them to liquefy and then further heat them to obtain amorphous powder.
We can find some mixed alum also, the alums with different monovalent, bivalent or trivalent cations are known as mixed alums.
Alums play an important role in industrial processes. Mostly potassium alum is part of industries, it is used as mordant in dies and also in the purification of water or we can say it is used in water treatment.
Hence we can conclude that common alum is ${{\text{K}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}{\text{.A}}{{\text{l}}_{\text{2}}}{{\text{(S}}{{\text{O}}_{\text{4}}}{\text{)}}_{\text{3}}}{\text{.24}}{{\text{H}}_{\text{2}}}{\text{O}}$

So, the correct answer is Option A .

Note:
As we all know that mostly potassium alum is used in industries that’s why simply potassium alum is also called alum. It has a wide variety of uses in industrial processes. Lithium atoms due to their small size is not capable of forming alums.