Which of the following is known as pseudo alum?

Options

A. `KMn(SO_(4))_(2).12H_(2)O`
B. `KCr(SO_(4))_(2).12H_(2)O`
C. `NH_(4)Fe(SO_(4))_(2).12H_(2)O`
D. `FeSO_(4).Al_(2)(SO_(4))_(3).24H_(2)O`

Answer

D

Explanation

Alums are double salts of the general formula
`H_(2)SO_(4) . R_(2)(SO_(4))_(3) . 24H_(2)O`
where `M` is a univalent ion such as `Na^(+) , K^(+)`, or `NH_(4)^(+)` and `R` is a trivalent ion such as `Al^(3+)` or `Cr^(3+)` . On the other hand, pseudo alum has the general formula
`MSO_(4) . R_(2)(SO_(4))_(3) . 24H_(2)O`
where `M` is a divalent ion such as `Fe^(2+)`. Therefore, `FeSO_(4) . Al_(2)(SO_(4))_(3) . 24H_(2)O` is a pseudo alum.