Question

# Which of the following is pseudo alum?A.${(N{H_4})_2}S{O_4} \cdot F{e_2}{(S{O_4})_3} \cdot 24{H_2}O$B.${K_2}S{O_4} \cdot A{l_2}{(S{O_4})_3} \cdot 24{H_2}O$C.$MnS{O_4} \cdot A{l_2}{(S{O_4})_3} \cdot 24{H_2}O$D.None of these

Hint:Pseudo Alum is a type of alum that is obtained when the regular monovalent metal that is present in true alum, is replaced by some other bivalent metal to form a new compound, i.e. pseudo alum.

The chemical formula for true alum can be given as $KAlS{O_4} \cdot A{l_2}(S{O_4}) \cdot 12{H_2}O$. Upon observation we can determine the monovalent metals present in the molecular structure of true alum are potassium (K) and aluminium (Al).
The chemical formula of pseudo alum can be identified as $AS{O_4} \cdot {B_2}{(S{O_4})_3} \cdot 24{H_2}O$ where A is a bivalent metal ion and B is a trivalent metal ion. For formation of different pseudo alums, A can be substituted with any bivalent metal ion. This includes all metal ions which are capable of +2 oxidation state.
In the options that are given to us, the options for substitution for the bivalent metal ions are: $N{H_4}$, potassium and manganese. Out of these options, only manganese is a bivalent metal ion.
Hence, the chemical formula for pseudo alum is $MnS{O_4} \cdot A{l_2}{(S{O_4})_3} \cdot 24{H_2}O$