Question:medium

Ferric alum has the composition (NH\(_4\))\(_2\)SO\(_4\)·Fe\(_2\)(SO\(_4\))\(_3\)·xH\(_2\)O. The value of x is

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Ferric alum = Mohr's salt with Fe\(^{3+}\) instead of Fe\(^{2+}\).
Updated On: Jun 16, 2026
  • 7
  • 24
  • 6
  • 15
Show Solution

The Correct Option is B

Solution and Explanation

The question asks for the value of x in the composition of Ferric alum given by the formula (NH\(_4\))\(_2\)SO\(_4\)·Fe\(_2\)(SO\(_4\))\(_3\)·xH\(_2\)O.

To understand the problem, we recognize that Ferric alum is a type of alum. Generally, alums are double sulfate salts, usually with the formula \(M_2SO_4 \cdot M_2'(SO_4)_3 \cdot 24H_2O\), where 24 is the number of water molecules of hydration, which is common among alums.

Let's analyze the given compound in the question: (NH\(_4\))\(_2\)SO\(_4\)·Fe\(_2\)(SO\(_4\))\(_3\)·xH\(_2\)O. The hydrate portion (xH\(_2\)O) is what determines the "x" value as it signifies the number of water molecules attached to the alum complex.

For typical alums, the number of water molecules of crystallization is usually 24. Ferric alum is no exception, conforming to this standard as it retains 24 water molecules in its crystalline structure.

Therefore, the correct value of x is 24.

To rule out other options:

  • 7: Uncommon in alum compositions and doesn't correspond to the usual structure for alums.
  • 6: Similarly insignificant for the standard hydrate number in alum chemistry.
  • 15: Is also incorrect as it doesn't align with the typical 24-hydrate structure of alums.

Hence, the value of x in Ferric alum is 24.

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