JEE Main 2026 — Chemistry Physical Chemistry

2026integereasy

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 mMHA solution is $\_\_\_\_$ .(Nearest integer)

[Given: The degree of dissociation can be neglected with respect to unity]

Official previous-year question

Held on 28 Jan 2026 · Verified 6 Jul 2026.

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Solution

For the weak acid dissociation $HA \rightleftharpoons H^+ + A^-$ with pKa = 4

We have Ka = 10⁻⁴.

For a 10 mM (0.01 M) solution where the degree of dissociation is negligible with respect to unity, the concentration of HA remains approximately 0.01 M.

The equilibrium expression gives

$Ka = \frac{[H^+]^2}{[HA]}$

Substituting: $10^{-4} = \frac{[H^+]^2}{0.01}$ , which yields $[H^+]^2 = 10^{-6}$ , so $[H^+] = 10^{-3}$ M.

Therefore, pH = $-\log(10^{-3}) = 3$ .

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JEE Main 2026 — Chemistry Physical Chemistry

2026integereasy

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 mMHA solution is $\_\_\_\_$ .(Nearest integer)

[Given: The degree of dissociation can be neglected with respect to unity]

Official previous-year question

Held on 28 Jan 2026 · Verified 6 Jul 2026.

Did you get this right?

Solution

For the weak acid dissociation $HA \rightleftharpoons H^+ + A^-$ with pKa = 4

We have Ka = 10⁻⁴.

For a 10 mM (0.01 M) solution where the degree of dissociation is negligible with respect to unity, the concentration of HA remains approximately 0.01 M.

The equilibrium expression gives

$Ka = \frac{[H^+]^2}{[HA]}$

Substituting: $10^{-4} = \frac{[H^+]^2}{0.01}$ , which yields $[H^+]^2 = 10^{-6}$ , so $[H^+] = 10^{-3}$ M.

Therefore, pH = $-\log(10^{-3}) = 3$ .

Your note

JEE Main Physical Chemistry in other years

Keep practicing Physical Chemistry

Work through every JEE Main Physical Chemistry PYQ, year by year.

Browse all Chemistry topics

JEE Main 2026 — Chemistry Physical Chemistry

Did you get this right?

Solution

Your note

JEE Main Physical Chemistry in other years

More JEE Main Physical Chemistry questions

Other Chemistry topics for JEE Main

Organic Chemistry

Inorganic Chemistry

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JEE Main 2026 — Chemistry Physical Chemistry

Did you get this right?

Solution

Your note

JEE Main Physical Chemistry in other years

More JEE Main Physical Chemistry questions

Other Chemistry topics for JEE Main

Organic Chemistry

Inorganic Chemistry

Keep practicing Physical Chemistry