Question

Which of the following is an alum?

• A. $FeSO _{4}\left( NH _{4}\right)_{2} SO _{4} \cdot 6 H _{2} O$
• B. $NaAlO _{2}$
• C. $Na _{2} SO _{4} \cdot Al _{2}\left( SO _{4}\right)_{3} \cdot 24 H _{2} O$
• D. $KCl \cdot MgCl _{2} \cdot 6 H _{2} O$

Answer 1

Answer: (C)

$Na _{2} SO _{4} \cdot Al _{2}\left( SO _{4}\right)_{3} \cdot 24 H _{2} O$

Answer 2

The term alum is used for all the double sulphates having the composition, $M _{2} SO _{4} .M _{2}'\left( SO _{4}\right)_{3}$ $24 H _{2} O$, where $M$ stands for monovalent basic radicals such $Na ^{+}, K ^{+}$ etc. and $M '$ for trivalent basic radicals such as $A l ^{3+}, Cr ^{3+}$ etc.