If the Kb values of ammonia, methylamine and ethylamine in aqueous solution are x, y and z respectively, which of the following relations is correct?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → y < z
The larger the Kb (the smaller the pKb), the stronger the base, since pKb = −log Kb.
pKb values in water: ammonia 4.75, methylamine (methanamine) 3.38, ethylamine (ethanamine) 3.29.
Order of pKb: ammonia > methylamine > ethylamine.
Order of Kb (reverse): ethylamine > methylamine > ammonia, so z > y > x.
Why the other options are incorrect
Option (1) x > z — Incorrect. It puts ammonia above ethylamine; in fact z > x.
Option (2) x > y — Incorrect. It puts ammonia above methylamine; in fact y > x.
Option (3) x > y > z — Incorrect. It puts ammonia above both amines; in fact z > y > x.
NCERT (2026-27 edition):
9.6 Chemical Reactions – “… Larger the value of Kb or smaller the value of pKb, stronger is the base. The pKb values of few amines are given in Table 9.3.”
“pKb value of ammonia is 4.75. Aliphatic amines are stronger bases than ammonia due to +I effect of alkyl groups leading to high electron density on the nitrogen atom.”
Table 9.3: pKb Values of Amines in Aqueous Phase
