Q.(a)

(a) Metal excess defects arise when an anion is missing and an electron fills the resulting hole (an F-centre), producing colour; a pseudo first-order reaction is genuinely higher order but appears first order because one reactant's concentration barely changes. OR (b) applying the weak-acid approximation $[H^+]=\sqrt{K_aC}$ to 0.1 M acetic acid gives $pH\approx2.87$.

(a)(i) Metal excess defect with an example: this non-stoichiometric point defect arises when an anion is missing from its regular lattice site, leaving a vacancy. To maintain overall electrical neutrality, an electron occupies this anion vacancy in place of the missing negative ion. Such an electron-occupied vacancy is called an F-centre (from German Farbzentrum, colour centre), and it is responsible for imparting colour to the otherwise colourless crystal, since the trapped electron can absorb visible light and get excited. Example: when colourless $NaCl$ crystals are heated in an atmosphere of sodium vapour, excess $Na$ atoms deposit on the crystal surface; $Cl^-$ ions diffuse to the surface and combine with these $Na$ atoms (forming $NaCl$), releasing electrons that diffuse into the crystal and occupy the resulting anion vacancies as F-centres. This causes the crystal to acquire a yellow colour.

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