Consider the dissociation equilibrium of the following weak acid $\mathrm{HA} \rightleftharpoons \mathrm{H}^{+}(\mathrm{aq})+\mathrm{A}^{-}(\mathrm{aq})$ If the pKa of the acid is 4, then the pH of 10 mM HA solution is ______. (Nearest integer) [Given : The degree of dissociation can be neglected with respect to unity]

JEE Main · 2026 · 28 Jan 2026 · Shift 1 · Equilibrium

Consider the dissociation equilibrium of the following weak acid

HA ⇌ H^{+} (aq) + A^{-} (aq)

If the pKa of the acid is 4, then the pH of 10 mM HA solution is ______. (Nearest integer)
[Given : The degree of dissociation can be neglected with respect to unity]

Answer: 3

Solution

For a weak acid whose dissociation is negligible compared with unity,

[H X^{+}] = K_{a} c

, so

pH = \frac{1}{2} (p K_{a} - lo g c)

. With

p K_{a} = 4

and

c = 10 mM = 1 0^{- 2}

pH = \frac{1}{2} (4 - lo g 1 0^{- 2}) = \frac{1}{2} (4 + 2) = 3

. The pH of the solution is 3.

JEE Main · 2026 · 28 Jan 2026 · Shift 1 · Integer answer

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