JEE Main 2026 — Chemistry Physical Chemistry

2026integermedium

Molar conductivity of a weak acid HQ of concentration 0.18 M was found to be $1 / 30$ of the molar conductivity of another weak acid HZ with concentration of 0.02 M. If $\lambda^{\circ} \mathrm{Q}^{-}$ happened to be equal with $\lambda^{\circ} \mathrm{Z}^{-}$ , then the difference of the $\mathrm{pK}_{\mathrm{a}}$ values of the two weak acids $\left(\mathrm{pK}_{\mathrm{a}}(\mathrm{HQ})-\mathrm{pK}_{\mathrm{a}}(\mathrm{HZ})\right)$ is $\_\_\_\_$ (Nearest integer).

[Given: degree of dissociation $(\alpha) \ll 1$ for both weak acids, $\lambda^{\circ}$ : limiting molar conductivity of ions]

Official previous-year question

Held on 24 Jan 2026 · Verified 6 Jul 2026.

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Solution

For weak acids with $\lambda^{\infty}Q^- = \lambda^{\infty}Z^-$ , the molar conductivity is $\lambda_m = \alpha \lambda^{\infty}$ .

From $\lambda_m^Q = \frac{1}{30}\lambda_m^Z$ , we get $\alpha_Q = \frac{\alpha_Z}{30}$ .

Using $K_a = \alpha^2 C$ for weak acids: $K_a(HQ) = \alpha_Q^2 \times 0.18 = \frac{\alpha_Z^2 \times 0.18}{900}$ and $K_a(HZ) = \alpha_Z^2 \times 0.02$ .

Therefore $\frac{K_a(HZ)}{K_a(HQ)} = \frac{0.02 \times 900}{0.18} = 100$ .

Thus $pK_a(HQ) - pK_a(HZ) = \log(100) = 2$ .

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JEE Main 2026 — Chemistry Physical Chemistry

2026integermedium

[Given: degree of dissociation $(\alpha) \ll 1$ for both weak acids, $\lambda^{\circ}$ : limiting molar conductivity of ions]

Official previous-year question

Held on 24 Jan 2026 · Verified 6 Jul 2026.

Did you get this right?

Solution

For weak acids with $\lambda^{\infty}Q^- = \lambda^{\infty}Z^-$ , the molar conductivity is $\lambda_m = \alpha \lambda^{\infty}$ .

From $\lambda_m^Q = \frac{1}{30}\lambda_m^Z$ , we get $\alpha_Q = \frac{\alpha_Z}{30}$ .

Using $K_a = \alpha^2 C$ for weak acids: $K_a(HQ) = \alpha_Q^2 \times 0.18 = \frac{\alpha_Z^2 \times 0.18}{900}$ and $K_a(HZ) = \alpha_Z^2 \times 0.02$ .

Therefore $\frac{K_a(HZ)}{K_a(HQ)} = \frac{0.02 \times 900}{0.18} = 100$ .

Thus $pK_a(HQ) - pK_a(HZ) = \log(100) = 2$ .

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JEE Main Physical Chemistry in other years

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JEE Main 2026 — Chemistry Physical Chemistry

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JEE Main 2026 — Chemistry Physical Chemistry

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Solution

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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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