13.
Which compound is most acidic?
a. H2O
b. H2S
c. H2Se
d. NH3

The most acidic compound is H₂Se (option c). Among H₂O, H₂S and H₂Se, acidity increases down the group: H₂O < H₂S < H₂Se, and NH₃ is much less acidic than any of these hydrides.​​

Introduction

In inorganic chemistry, comparing the acidity of hydrides like H₂O, H₂S, H₂Se and the covalent base NH₃ is a common conceptual multiple-choice question. The key lies in understanding how acidity trends in group 16 hydrides change down the group and why NH₃, a group 15 hydride, behaves predominantly as a base rather than an acid.​

Concept: acidity of group 16 hydrides

• The hydrides of group 16 are H₂O, H₂S, H₂Se, H₂Te, and their acidic character increases down the group:
  H₂O < H₂S < H₂Se < H₂Te.​

• This happens because the E–H bond (E = O, S, Se, Te) becomes weaker as the size of the chalcogen atom increases, so the proton (H⁺) is released more easily, strengthening the acid.​

Approximate pKₐ values reflect this trend: H₂O ≈ 14, H₂S ≈ 7, H₂Se ≈ 3–4, showing that H₂Se is much stronger as an acid than H₂S or H₂O.​​

Option-wise explanation

Option a: H₂O

• Water can act as a very weak acid (autoionization: H₂O ⇌ H⁺ + OH⁻), but its pKₐ is very high (≈14), meaning it donates a proton only to a very small extent in water.​​

• Among group 16 hydrides, H₂O is the least acidic: H₂O < H₂S < H₂Se < H₂Te.​
  Therefore, H₂O is not the most acidic; it is actually the weakest acid among the three chalcogen hydrides listed.

Option b: H₂S

• H₂S is a weak diprotic acid with pKₐ₁ ≈ 7, significantly more acidic than H₂O but still considerably weaker than H₂Se.​​

• The S–H bond is weaker than the O–H bond, so proton donation is easier than in water, giving the order H₂O < H₂S in acidity.​
  However, acidity continues to increase down the group, so H₂S is less acidic than H₂Se.

Option c: H₂Se (correct)

• H₂Se has an even larger central atom (Se) with a weaker Se–H bond than the S–H bond in H₂S, which makes H⁺ loss easier and increases acid strength.​

• Literature and data on group 16 hydrides consistently show the acidity increasing as H₂O < H₂S < H₂Se < H₂Te, so among the three hydrides in the options, H₂Se is the strongest acid.​
  Thus, H₂Se is the most acidic compound in the given list, making option c correct.

Option d: NH₃

• NH₃ is a group 15 hydride and is classically a Lewis base / Brønsted base, because the nitrogen lone pair readily accepts a proton to form NH₄⁺.​

• Its conjugate acid NH₄⁺ has pKₐ ≈ 9.2, which corresponds to NH₃ being a weak base, not a useful proton donor; NH₃ itself is extremely weak as an acid compared with H₂O, H₂S or H₂Se.​​
  Therefore, NH₃ is the least acidic species here and cannot be the most acidic compound among the options.

Final answer

Among H₂O, H₂S, H₂Se and NH₃, H₂Se is the most acidic compound, so the correct option is:
c. H₂Se.​​