Consider the following sequence of reactions to produce major product (A)
Molar mass of product(A) isg mol⁻¹.; (Given molar mass ing mol⁻¹ ofC:12,H:1,O:16,Br:80, N:14,P:31)

Solution

Chemistry ma’am · 4 steps

The nitrogen goes in only to leave: NO2 becomes NH2, then N2, then plain H, so A is just a bromotoluene.

1. Brominate the ring. Br₂/Fe puts one Br on the ring. CH₃ is the activating group, so it directs Br ortho or para to itself, away from the deactivating NO₂ group's influence. Either way, the ring now carries CH₃, NO₂ and one Br.
2. Reduce, then diazotise. Sn/HCl reduces -NO₂ to -NH₂. NaNO₂/HCl at 273 K turns that -NH₂ into the diazonium group -N₂⁺Cl⁻.
3. Remove the nitrogen. H₃PO₂ in water reduces ArN₂⁺ to Ar–H and releases N₂. The position that held NO₂ now holds only H, so A is a bromotoluene, C₇H₇Br. Whichever ring position the Br took, the formula is the same.

   M(C₇H₇Br) = 7(12) + 7(1) + 80 = 84 + 7 + 80 = 171 g mol⁻¹

4. So. The molar mass of A is 171 g mol⁻¹. The masses of N and P are given only as distractors, because neither element ends up in A.

The common slip: 187. Treating H₃PO₂/H₂O as hydrolysis to a phenol gives bromocresol, C₇H₇BrO, with mass 187. H₃PO₂ replaces the diazonium group with H, not OH.

The idea to keep. NO₂ → NH₂ → N₂⁺ →(H₃PO₂) H is a removable directing group. Use it to place a substituent, then delete it. In a molar mass question, write the final formula first.

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