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JEE Main levelSingle correct optionDifficulty 2 of 3pb023

Which of the following is NOT an alum?

A $(NH X_{4}) X_{2} SO X_{4} \cdot Fe X_{2} (SO X_{4}) X_{3} \cdot 24 H X_{2} O$
B $K X_{2} SO X_{4} \cdot Al X_{2} (SO X_{4}) X_{3} \cdot 24 H X_{2} O$
C $K X_{2} SO X_{4} \cdot Cr X_{2} (SO X_{4}) X_{3} \cdot 24 H X_{2} O$
D $FeSO X_{4} \cdot (NH X_{4}) X_{2} SO X_{4} \cdot 6 H X_{2} O$ (Mohr's salt)

Answer: D

Alums have the general formula

M X_{2} SO X_{4} \cdot N X_{2} (SO X_{4}) X_{3} \cdot 24 H X_{2} O

with a monovalent M and a trivalent N. Mohr's salt contains a divalent iron(II) and only six water molecules, so it is a different double salt (of the vitriol type).

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