Q.(a) Derive an expression for Ostwald's Dilution Law. OR

(a) Ostwald's dilution law derives $K_a\approx C\alpha^2$ for a weak electrolyte, showing $\alpha \propto 1/\sqrt{C}$ (dissociation rises on dilution). (b) Aniline's lone pair is tied up by AlCl$_3$, killing Friedel-Crafts reactivity; nylon-2-nylon-6 is a glycine/6-aminohexanoic-acid copolymer.

(a) Derivation of Ostwald's Dilution Law: Consider a weak, binary electrolyte $AB$ ionising in solution: $AB \rightleftharpoons A^+ + B^-$ Let the initial concentration be $C$ (mol/L) and the degree of dissociation be $\alpha$. At equilibrium: $[AB]=C(1-\alpha)$, $[A^+]=C\alpha$, $[B^-]=C\alpha$. The dissociation (ionisation) constant is: $K_a=\frac{[A^+][B^-]}{[AB]}=\frac{(C\alpha)(C\alpha)}{C(1-\alpha)}=\frac{C\alpha^2}{1-\alpha}$ For a weak electrolyte, $\alpha \ll 1$, so $(1-\alpha)\approx1$, giving the approximate form: $K_a \approx C\alpha^2 \quad\Rightarrow\quad \alpha \approx \sqrt{\frac{K_a}{C}}=\sqrt{K_a V}$ (using $C=1/V$, where $V$ is the volume containing 1 mole). This is Ostwald's dilution law: since $K_a$ is constant at a given temperature, $\alpha$ increases as $C$ decreases (i.e. on dilution) — the degree of dissociation of a weak electrolyte increases with dilution.

OR (b)(i) Why aniline does not undergo Friedel-Crafts reaction: Friedel-Crafts alkylation/acylation requires a Lewis acid catalyst, typically anhydrous $AlCl_3$. Aniline's -NH$_2$ group carries a lone pair of electrons on nitrogen, which is a good Lewis base; this lone pair immediately reacts with (donates to) the Lewis acid catalyst $AlCl_3$ to form a salt-like complex, $C_6H_5-\overset{+}{N}H_2-AlCl_3^-$. Once complexed this way, the nitrogen becomes positively charged, strongly withdrawing electron density from the ring by induction, and the aromatic ring becomes deactivated and effectively meta-directing — the exact opposite of what's needed for electrophilic Friedel-Crafts substitution. Because the required Lewis acid catalyst is consumed/deactivated by the amine itself (rather than activating an electrophile), the Friedel-Crafts reaction fails for aniline.

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