Arrange the following labelled hydrogens in decreasing order of acidity :

Solution

Chemistry ma’am · 4 steps

Judge each acid by its conjugate base: the better the negative charge is spread out, the more acidic the H.

1. Carboxylic acid wins. Removing H_b gives a carboxylate ion. Its negative charge is shared equally over two oxygen atoms, so it is the most stable base here. That makes H_b the most acidic, with pKₐ ≈ 4.5.
2. Where is the nitro. The NO₂ group is para to the OH marked c and meta to the OH marked d. In a phenoxide ion the negative charge spreads onto the ortho and para carbons. So only the para NO₂ can pull that charge onto its own oxygens by resonance.

   Hydrogen	Conjugate base	Stabilised by	Approx. pKₐ
   b	carboxylate	resonance over two O atoms	about 4.5
   c	phenoxide, NO₂ para	ring resonance plus -R and -I of NO₂	about 7
   d	phenoxide, NO₂ meta	ring resonance plus only -I of NO₂	about 8.4
   a	acetylide	charge held on an sp carbon only	about 25

3. Terminal alkyne last. The C≡C-H hydrogen is acidic only because an sp carbon holds negative charge fairly well. The charge stays on carbon, a much weaker home than oxygen, so Hₐ is far less acidic than any OH.
4. So. The decreasing order of acidity is b > c > d > a, which is option (a).

The idea to keep. For acidity, first ask which atom holds the charge (O beats C), then how far it spreads. An NO₂ group pulls charge by resonance only from ortho or para positions; at meta it helps only by induction.

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