Suitable reagents for the laboratory preparation of nitrogen are

Analyze the question: We need the pair of reagents used in the laboratory to prepare nitrogen gas.

Concept: The standard laboratory preparation of nitrogen involves heating a solution of ammonium nitrite, which decomposes to give nitrogen and water. Ammonium nitrite is unstable and is not stored, so it is made in the flask by mixing:

• sodium nitrite, NaNO₂ (sodium dioxonitrate(III))
• ammonium chloride, NH₄Cl

Solution:

1. Double decomposition forms ammonium nitrite: NaNO₂(aq) + NH₄Cl(aq) → NH₄NO₂(aq) + NaCl(aq)

2. Gentle heating decomposes the ammonium nitrite: NH₄NO₂(aq) → N₂(g) + 2H₂O(l)

Checking the options:

• A. Sodium dioxonitrate(III) (NaNO₂) with ammonium chloride (NH₄Cl) gives NH₄NO₂, then N₂. This is correct.
• B. Sodium trioxonitrate(V) is NaNO₃, a nitrate. Combined with NH₄Cl, it would give ammonium nitrate, which decomposes to N₂O (laughing gas), not N₂.
• C. NaCl with NH₄NO₃ gives no suitable reaction for nitrogen. Heating NH₄NO₃ gives N₂O.
• D. NaCl with ammonium nitrite would work only if the ammonium nitrite were available as a reagent. It is unstable and not a standard bench reagent, and the pairing is not the standard method. Option A is the standard choice.

Answer: A.

Exam tip: Remember the oxidation-state naming: dioxonitrate(III) is nitrite (NO₂⁻) and trioxonitrate(V) is nitrate (NO₃⁻). For nitrogen gas, think "nitrite + ammonium", because ammonium nitrite breaks down to N₂ + 2H₂O. Ammonium nitrate gives N₂O instead.