JEE Main 2026 · 2 Apr · Shift 2 · Chemistry Q56

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ChemistrySingle correct+4 / -1Question 56 of 75

At 25 °C, 20.0 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 : $K_a=5\times10^{-4}$ , $pK_a=3.3$ , $\alpha\ll1$

(a) 0.7, (b) 2.0

(a) 2.0, (b) 3.3

(a) 1.1, (b) 2.2

(a) 3.0, (b) 2.2

Answer and worked solution

Official answer: B

(a) Start. $[\ce{H+}]=\sqrt{K_aC}=\sqrt{5\times10^{-4}\times0.2}=10^{-2}$ , so pH = 2.0.

(b) 10 mL NaOH is half the 20 mL needed, so it is the half-equivalence point: $[\ce{HX}]=[\ce{X-}]$ and pH = p $K_a$ = 3.3.

(B)

Topics:Buffer solutions Indicators and titrations

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JEE Main 2026 · 2 Apr · Shift 2, Chemistry question 56