"A" obtained by Ostwald's method involving air oxidation of NH₃, upon further air oxidation produces "B", "B" on hydration forms an oxoacid of Nitrogen along with evolution of "A". The oxoacid also produces "A" and gives positive brown ring test.
Identify A and B, respectively :

Solution

Chemistry ma’am · 5 steps

Walk through Ostwald's process step by step: NO first, then NO2, and NO comes back when NO2 meets water.

1. Air oxidation of ammonia. In Ostwald's process, NH₃ is burnt in air over a Pt/Rh gauze at about 500 K and 9 bar. The product is nitric oxide, so A is NO: 4NH₃ + 5O₂ → 4NO + 6H₂O.
2. Further oxidation. NO combines readily with more oxygen from air to give brown nitrogen dioxide, so B is NO₂: 2NO + O₂ → 2NO₂.
3. Hydration of B. NO₂ dissolves in water and disproportionates. Nitrogen goes from +4 to +5 in HNO₃ and to +2 in NO, so NO (that is, A) is given off again.

   3NO₂ + H₂O → 2HNO₃ + NO

4. Check the oxoacid. The oxoacid is HNO₃. When it is reduced, for example by Fe²⁺ in the brown ring test, it gives NO. That NO forms the brown complex [Fe(H₂O)₅(NO)]²⁺, so every clue matches.
5. So. A is NO and B is NO₂, which is option (d).

The idea to keep. Remember the chain for Ostwald's process: NH₃ → NO → NO₂ → HNO₃. When NO₂ is absorbed in water, one third of it returns as NO, and that NO is recycled.

Answer from the NTA final answer key. Solution by PrepWiser.