Chirkut.বাংলা

MCQ practice · SSC Chemistry Board questions · Chapter 10: Mineral Resources: Metals and Non-metals

Dhaka Board 2022

Which is the formula of rust?

Think it through first, then pick an option.

  1. a FeO.3H₂O
  2. b Fe₂O₃.H₂O
  3. c Fe₂O₃.2H₂O
  4. d Fe₂O₃.nH₂O
Answer and explanation

Correct answer: d Fe₂O₃.nH₂O

Rust is hydrated ferric oxide, Fe₂O₃.nH₂O.

Why the other options are wrong

  • a FeO.3H₂O: FeO is ferrous oxide; rust is ferric oxide, Fe₂O₃.nH₂O.
  • b Fe₂O₃.H₂O: The book writes the number of water molecules as n, not a fixed 1: Fe₂O₃.nH₂O.
  • c Fe₂O₃.2H₂O: The number of water molecules is not fixed, so it is n: Fe₂O₃.nH₂O.