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Alkali metal sulphates generally crystallize as double salts with trivalent metal sulphates, formings Block Elements Chemistry Question

Question

Alkali metal sulphates generally crystallize as double salts with trivalent metal sulphates, forming compounds known as Alums. Which of the following alkali metals is the EXCEPTION and does NOT form an alum?

Answer: C

💡 Solution & Explanation

Alums have the general formula $M_2SO_4 \cdot M'_{2}(SO_4)_3 \cdot 24H_2O$. Lithium sulphate ($Li_2SO_4$) does not form alums because the $Li^+$ ion is too small to fit properly into the highly specific, rigid crystal lattice structure required for alum formation.

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