p-Block Elements (Groups 13-14), on the exam screen.

Oxygen shows several oxidation states besides -2: it is -1 in peroxides (H$_{2}$O$_{2}$), -½ in superoxides, and positive (+2, +1) in fluorides such as OF$_{2}$ and O$_{2}$F$_{2}$, so statement (4) is incorrect and is the required answer. The other statements are correct: carbon readily forms p$\pi$-p$\pi$ bonds (C=C, C≡C); BCl$_{3}$ is monomeric (B uses back-bonding) while AlCl$_{3}$ dimerises to Al$_{2}$Cl$_{6}$; and catenation decreases down Group 14 as C >> Si > Ge $\approx$ Sn.

Potash alum is the double salt $\mathrm{K}_{2}\mathrm{SO}_{4}.\mathrm{Al}_{2}(\mathrm{SO}_{4})_{3}.24\mathrm{H}_{2}\mathrm{O}$, most compactly written as $\mathrm{KAl}(\mathrm{SO}_{4})_{2}.12\mathrm{H}_{2}\mathrm{O}$. Each formula unit supplies one $\mathrm{K}^{+}$ balanced against two sulphate ions (together with one $\mathrm{Al}^{3+}$), so the ratio is 1 : 2. NCERT Class 11, The p-Block Elements (compounds of aluminium).

Boron sits in the second period, where only one 2s and three 2p orbitals are available for bonding - four orbitals in all - and there are no 2d orbitals to expand into. Its covalency therefore cannot exceed four, as in $\mathrm{BF}_{4}^-$ and $[\mathrm{BH}_{4}]^-$. NCERT Class 11, The p-Block Elements (anomalous properties of boron).

CO$_{2}$ gas is not the refrigerant used here; solid CO$_{2}$ (dry ice) is. C$_{60}$ contains twelve five-membered and twenty six-membered rings, so (Statement II) is false. ZSM-5 converts alcohols to gasoline and CO is colourless and odourless, so only (Statement III) and (Statement IV) are correct. Chapter: p-Block Elements (Groups 13-14).