Which of the following correctly represents the chemical formula of a compound formed by aluminium and sulphate ions?

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RRB JE ECE 22 Apr 2025 Shift 1 CBT 2 Official Paper
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  1. Al(SO4)3
  2. Al2SO4
  3. Al2(SO4)3
  4. Al3(SO4)2

Answer (Detailed Solution Below)

Option 3 : Al2(SO4)3
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The correct answer is Al2(SO4)3.

Key Points

  • Aluminium has a valency of +3 and forms the ion Al3+.
  • Sulphate has a valency of -2 and forms the ion SO42-.
  • To form a neutral compound, the charges must balance. Therefore, two Al3+ ions (total charge +6) will balance three SO42- ions (total charge -6).
  • Thus, the correct formula for the compound formed by aluminium and sulphate ions is Al2(SO4)3.
  • This compound is commonly known as aluminium sulphate, which is used in water purification, paper manufacturing, and as a mordant in dyeing and printing textiles.

 Additional Information

  • Al(SO4)3
    • This formula incorrectly suggests that one aluminium ion can combine with three sulphate ions, which does not balance the charges correctly.
  • Al2SO4
    • This formula incorrectly represents the compound by not indicating the correct ratio of aluminium to sulphate ions.
  • Al3(SO4)2
    • This formula incorrectly represents the compound by suggesting a different ratio that does not balance the charges of the ions correctly.
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