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Amines Set-1 MCQ Online Practice Test | Class 12 | Quiz Tweek

Amines Set-1

Practice Important MCQs for Amines — Chemistry, Class 12.

Amines are a high-scoring and frequently tested topic in Class 12 Chemistry because they connect reaction mechanisms, basicity/acid-base behavior, resonance effects, and major synthetic strategies. Board exams and competitive tests commonly ask about how substitution (alkyl/aryl, primary/secondary/tertiary), pKa–pKb relations, and electrophilic aromatic substitution directing effects determine basicity and product formation—so mastering these questions directly improves both concept clarity and speed.

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Q1. Given the conjugate-acid $pK_a(CH_3NH_3^+)=10.6$ and $pK_a(C_6H_5NH_3^+)=4.6$ , which of the following is correct about $pK_b$ and basicity?

(A)

$pK_b(CH_3NH_2)=3.4$ and $pK_b(C_6H_5NH_2)=9.4$ ; aniline ( $C_6H_5NH_2$ ) is the stronger base.

(B)

$pK_b(CH_3NH_2)=10.6$ and $pK_b(C_6H_5NH_2)=4.6$ .

(C)

$pK_b(CH_3NH_2)=3.4$ and $pK_b(C_6H_5NH_2)=9.4$ ; methylamine ( $CH_3NH_2$ ) is the stronger base.

(D)

$pK_b(CH_3NH_2)=9.4$ and $pK_b(C_6H_5NH_2)=3.4$ .

Q2. Which of the following amines is the most basic in aqueous solution?

(A)

Cyclohexylamine, $C_6H_{11}NH_2$ .

(B)

p-Nitroaniline, $p$ -NO $_2$ -C $_6$ H $_4$ NH $_2$ .

(C)

N-Methylaniline, $C_6H_5NHCH_3$ .

(D)

Aniline, $C_6H_5NH_2$ .

Q3. Equal initial concentrations of methylamine and aniline are present in a solution at pH 6. Given $pK_a(CH_3NH_3^+)=10.6$ and $pK_a(C_6H_5NH_3^+)=4.6$ , approximately how many times more methylammonium ions ( $CH_3NH_3^+$ ) are present than anilinium ions ( $C_6H_5NH_3^+$ )?

(A)

$\approx 260$ times.

(B)

$\approx 6$ times.

(C)

$\approx 0.038$ times.

(D)

$\approx 26$ times.

Q4. Statement I: In the gas phase triethylamine $N(C_2H_5)_3$ is a stronger base than methylamine $CH_3NH_2$ , but in aqueous solution methylamine is the stronger base.
Statement II: This reversal occurs because in the gas phase intrinsic electron-donating effect of the ethyl groups increases basicity, whereas in aqueous solution solvation stabilizes the smaller methylammonium ion much more effectively than the sterically hindered triethylammonium ion.
Choose the correct option.

(A)

Both Statement I and Statement II are true, and Statement II correctly explains Statement I.

(B)

Both Statement I and Statement II are true, but Statement II does not correctly explain Statement I.

(C)

Statement I is true but Statement II is false.

(D)

Statement I is false and Statement II is true.

Q5. Starting from aniline ( $C_6H_5NH_2$ ), which sequence of reagents will give $m$ -nitroaniline ( $m$ -NO $_2$ -C $_6$ H $_4$ NH_2$) as the major product?

(A)

Direct nitration with $HNO_3/H_2SO_4$ .

(B)

(i) Acetylation: $Ac_2O$ (or $CH_3COCl$ ) to form anilide, (ii) Nitration: $HNO_3/H_2SO_4$ , (iii) Hydrolysis: $NaOH\,(aq)$ to regenerate the amino group.

(C)

(i) Protonation: $HCl$ (form anilinium salt), (ii) Nitration $HNO_3/H_2SO_4$ , (iii) Neutralization.

(D)

(i) Sulfonation: $SO_3/H_2SO_4$ , (ii) Nitration: $HNO_3/H_2SO_4$ , (iii) Desulfonation (heat/water).

Q6. Calculate the pH of a $0.10\ \mathrm{M}$ solution of methylamine $\,\mathrm{CH_3NH_2}\,$ at $25^\circ\mathrm{C}$ . Given $K_b(\mathrm{CH_3NH_2})=4.4\times10^{-4}$ . (Take $K_w=1.0\times10^{-14}$ if needed.)

(A)

$10.90$

(B)

$11.45$

(C)

$11.82$

(D)

$12.30$

Q7. Equal volumes of $0.10\ \mathrm{M}$ methylamine $\,\mathrm{CH_3NH_2}\,$ and $0.10\ \mathrm{M}$ aniline $\,\mathrm{C_6H_5NH_2}\,$ are mixed at $25^\circ\mathrm{C}$ . What is the pH of the resulting solution? Given $K_b(\mathrm{CH_3NH_2})=4.4\times10^{-4}$ and $K_b(\mathrm{C_6H_5NH_2})=4.3\times10^{-10}$ . (Assume volumes are additive and $K_w=1.0\times10^{-14}$ .)

(A)

$11.67$

(B)

$11.82$

(C)

$10.95$

(D)

$9.30$

Q8. You have $100.0\ \mathrm{mL}$ of $0.10\ \mathrm{M}$ methylamine $\,\mathrm{CH_3NH_2}\,$ . It is titrated with $0.10\ \mathrm{M}$ HCl. What volume of HCl must be added to obtain a solution of pH $9.00$ at $25^\circ\mathrm{C}$ ? Given $K_b(\mathrm{CH_3NH_2})=4.4\times10^{-4}$ and $K_w=1.0\times10^{-14}$ . (Neglect any volume change except by mixing.)

(A)

$25.0\ \mathrm{mL}$

(B)

$50.0\ \mathrm{mL}$

(C)

$110.0\ \mathrm{mL}$

(D)

$97.8\ \mathrm{mL}$

Q9. Arrange the following amines in order of decreasing basicity (most → least) in (i) the gas phase and (ii) aqueous solution (25°C): $\mathrm{NH_3}$ , $\mathrm{CH_3NH_2}$ , $\mathrm{(CH_3)_2NH}$ , $\mathrm{(CH_3)_3N}$ . Choose the correct pair of orders.

(A)

Gas: $\mathrm{(CH_3)_2NH}>\mathrm{(CH_3)_3N}>\mathrm{CH_3NH_2}>\mathrm{NH_3}$ ; Aqueous: $\mathrm{(CH_3)_3N}>\mathrm{(CH_3)_2NH}>\mathrm{CH_3NH_2}>\mathrm{NH_3}$

(B)

Gas: $\mathrm{(CH_3)_3N}>\mathrm{(CH_3)_2NH}>\mathrm{CH_3NH_2}>\mathrm{NH_3}$ ; Aqueous: $\mathrm{(CH_3)_2NH}>\mathrm{CH_3NH_2}>\mathrm{(CH_3)_3N}>\mathrm{NH_3}$

(C)

Gas: $\mathrm{(CH_3)_3N}>\mathrm{CH_3NH_2}>\mathrm{(CH_3)_2NH}>\mathrm{NH_3}$ ; Aqueous: $\mathrm{CH_3NH_2}>\mathrm{(CH_3)_2NH}>\mathrm{(CH_3)_3N}>\mathrm{NH_3}$

(D)

Gas: $\mathrm{NH_3}>\mathrm{CH_3NH_2}>\mathrm{(CH_3)_2NH}>\mathrm{(CH_3)_3N}$ ; Aqueous: $\mathrm{(CH_3)_2NH}>\mathrm{(CH_3)_3N}>\mathrm{CH_3NH_2}>\mathrm{NH_3}$

Q10. For the proton-transfer equilibrium

\mathrm{CH_3NH_3^+ + C_6H_5NH_2 \rightleftharpoons CH_3NH_2 + C_6H_5NH_3^+}

calculate the equilibrium constant $K$ at $25^\circ\mathrm{C}$ . Given $K_b(\mathrm{CH_3NH_2})=4.4\times10^{-4}$ , $K_b(\mathrm{C_6H_5NH_2})=4.3\times10^{-10}$ and $K_w=1.0\times10^{-14}$ .

(A)

$9.8\times10^{-7}$

(B)

$1.1\times10^{6}$

(C)

$0.99$

(D)

$2.3\times10^{-5}$

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