Given pKb values at 298 K: CH3NH2 3.38, (CH3)2NH 3.27, (CH3)3N 4.22, NH3 4.75, C6H5NH2 9.38. Which conclusions are correct?

A 0.01 M solution of (CH3)3N has a lower pH than a 0.01 M solution of CH3NH2.

(CH3)3NH+ is a weaker acid than NH4+.

A 0.01 M solution of (CH3)3N has a lower pH than a 0.01 M solution of CH3NH2.

K_b(CH3NH2) / K_b(C6H5NH2) is about 10^3.

(CH3)3NH+ is a weaker acid than NH4+.

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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3nc123

Given pKb values at 298 K: $CH X_{3} NH X_{2}$ 3.38, $(CH X_{3}) X_{2} NH$ 3.27, $(CH X_{3}) X_{3} N$ 4.22, $NH X_{3}$ 4.75, $C X_{6} H X_{5} NH X_{2}$ 9.38. Which conclusions are correct?

A $K_{b} ((CH X_{3}) X_{2} NH) > K_{b} (CH X_{3} NH X_{2})$
BA 0.01 M solution of $(CH X_{3}) X_{3} N$ has a lower pH than a 0.01 M solution of $CH X_{3} NH X_{2}$ .
C $K_{b} (CH X_{3} NH X_{2}) / K_{b} (C X_{6} H X_{5} NH X_{2})$ is about $1 0^{3}$ .
D $(CH X_{3}) X_{3} NH X^{+}$ is a weaker acid than $NH X_{4} X^{+}$ .

Answer: A, B, D

A smaller pKb means a larger Kb, so

3.27 < 3.38

makes (A) true. Trimethylamine (pKb 4.22) is a weaker base than methylamine (3.38), so its solution has the lower pH and (B) is true. The ratio of Kb values is

1 0^{(9.38 - 3.38)} = 1 0^{6}

, not

1 0^{3}

, so (C) is false. The conjugate acid pKa is 14 - pKb: 9.78 for the trimethylammonium ion against 9.25 for the ammonium ion, so it is a weaker acid and (D) is true.

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Given pKb values at 298 K: CHX3​NHX2​ 3.38, (CHX3​)X2​NH 3.27, (CHX3​)X3​N 4.22, NHX3​ 4.75, CX6​HX5​NHX2​ 9.38. Which conclusions are correct?

Answer: A, B, D

More questions on Basicity of amines

Given pKb values at 298 K: $CH X_{3} NH X_{2}$ 3.38, $(CH X_{3}) X_{2} NH$ 3.27, $(CH X_{3}) X_{3} N$ 4.22, $NH X_{3}$ 4.75, $C X_{6} H X_{5} NH X_{2}$ 9.38. Which conclusions are correct?