Chemistry · Ionic Equilibrium

JEE Main 2026 — 2 April, Evening Shift — Question 53

At $25^{\circ} \mathrm{C}, 20.0 \mathrm{~mL}$ of 0.2 M weak monoprotic acid HX is titrated against 0.2 M NaOH . The pH of the solution (a) at the start of the titration (when NaOH has not been added) and (b) when 10 mL of NaOH is added respectively, are : Given : $\mathrm{K}_{\mathrm{a}}=5 \times 10^{-4}, \mathrm{pK}_{\mathrm{a}}=3.3, \alpha \ll 1$

Option A:
$0.7,2.0$
Option B:
$2.0,3.3$
Correct
Option C:
$1.1,2.2$
Option D:
3.0, 2.2

Answer: B

Step-by-step solution

For $0.2 \mathrm{M}, 20 \mathrm{ml} \mathrm{HX}$ solution,

\begin{array}{rl} \mathrm{pH} \text { of weak acid }= & \frac{1}{2}\left[\mathrm{pk}_{\mathrm{a}}-\log \mathrm{c}\right] & =\frac{1}{2}[4-\log 5-\log 0.2] & =\frac{1}{2}[4-\log 5+\log 5] & =2 \mathrm{HX} & + 20 \mathrm{ml}, 0.2 \mathrm{M} & 10 \mathrm{ml}, 0.2 \mathrm{M} 4 \mathrm{~m} \text { mole } & 2 \mathrm{~m} \text { mole } 2 \mathrm{~m} \text { mole } & - 2 \mathrm{~m} \text { mole } \end{array}

$[\mathrm{HX}+\mathrm{NaA}] \Rightarrow$ acidic buffer

\begin{aligned} & \mathrm{pH}=\mathrm{pK}_{\mathrm{a}}+\log \frac{\text { salt }}{\text { acid }} & \mathrm{pH}=3.3+\log \frac{2}{2} & \mathrm{pH}=3.3 \end{aligned}

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Exam: JEE Main 2026
Paper: 2 April, Evening Shift
Subject: Chemistry
Chapter: Ionic Equilibrium
Topic: Solutions containing one Acid or Base

The first and second ionization constants of a weak dibasic acid H 2 A are 8.1 × 10^-8 and 1.0 × 10^-13 respectively. 0.1 mol of H 2 A was…

Answer: B

Step-by-step solution

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