Class 12 Boards Chemistry Practice Questions

B. Molarity — Molarity (M) = moles of solute per litre of solution. Molality is moles of solute per kilogram of solvent.

C. –COOH — Carboxylic acids contain the carboxyl group –COOH. –OH is alcohol, –CHO is aldehyde, and –NH₂ is amine.

A. Galvanisation — Galvanisation is coating iron/steel with zinc; the zinc corrodes preferentially (sacrificial protection), shielding the iron. Vulcanisation is a process for rubber.

C. 3 — HCl is a strong acid that fully ionises, so [H⁺] = 10⁻³ M. pH = −log[H⁺] = −log(10⁻³) = 3.

C. Hydrochloric acid — Hydrochloric acid (HCl) ionises completely in water and is a strong acid. Acetic, carbonic and citric acids ionise only partially and are weak acids.

B. −2 — Oxygen usually has an oxidation state of −2 (as in H₂O, CO₂). Exceptions include peroxides (−1, as in H₂O₂) and compounds with fluorine.

B. Depression in freezing point — Colligative properties depend only on the number of solute particles, not their nature. Depression in freezing point is one such property (along with relative lowering of vapour pressure, elevation of boiling point, and osmotic pressure).

C. 3 — K2SO4 dissociates into 2 K+ ions and 1 SO4^2- ion, giving 3 particles per formula unit, so i = 3 for complete dissociation.

B. Increases slightly and approaches a limiting value — For a strong electrolyte the number of ions is nearly constant; on dilution inter-ionic attractions decrease, so molar conductivity rises slightly and approaches the limiting molar conductivity (Λ°m).

C. It is independent of the initial concentration — For a first-order reaction t½ = 0.693/k, which contains no concentration term, so the half-life is independent of the initial concentration of the reactant.

B. mol L^-1 s^-1 — For a zero-order reaction, rate = k (rate has units mol L^-1 s^-1 and concentration term is raised to power zero), so k also has units of mol L^-1 s^-1.

A. d-d electronic transitions — In the presence of ligands the d-orbitals split; electrons absorb visible light to move between these split d-levels (d-d transitions), and the complementary colour is transmitted, giving the ion its colour.

D. A steady decrease in atomic and ionic radii across the lanthanoid series — As the atomic number increases across the lanthanoids, the added 4f electrons shield the nuclear charge poorly, so the effective nuclear charge felt by outer electrons rises and atomic/ionic radii decrease steadily.

B. +3 — NH3 is neutral, so the +3 charge balancing the three Cl- ions resides entirely on cobalt; hence Co is in the +3 oxidation state.

C. SCN- — An ambidentate ligand has two different donor atoms but binds through only one at a time. SCN- can coordinate through sulphur (thiocyanato) or nitrogen (isothiocyanato).

D. tert-butyl chloride — SN1 rate depends on the stability of the carbocation intermediate. tert-butyl chloride forms a tertiary carbocation, the most stable of the options, so it reacts fastest by SN1.

B. Phenol — Phenol's conjugate base (phenoxide) is stabilised by resonance delocalisation of the negative charge into the ring, whereas the alkyl alcohols form non-resonance-stabilised alkoxides; hence phenol is far more acidic.

C. Acetone — The iodoform test is positive for compounds containing the CH3CO- (methyl ketone) group or those oxidisable to it. Acetone (CH3COCH3) has this group; benzaldehyde and formaldehyde lack a CH3CO- group.

D. Benzaldehyde — The Cannizzaro reaction occurs only with aldehydes that have no alpha-hydrogen. Benzaldehyde has no alpha-H, so it disproportionates into benzyl alcohol and benzoate; the others all possess alpha-hydrogens.

C. Primary amines — Only primary amines (aliphatic or aromatic) react with chloroform and alcoholic KOH to give isocyanides (carbylamines) with an offensive smell; secondary and tertiary amines do not respond.

C. Sucrose — Sucrose on hydrolysis yields one molecule each of glucose and fructose, so it is a disaccharide. Glucose and fructose are monosaccharides and starch is a polysaccharide.

D. Peptide bond — Amino acids are linked by peptide (amide, -CO-NH-) bonds formed by condensation between the carboxyl group of one amino acid and the amino group of the next; glycosidic and phosphodiester bonds occur in carbohydrates and nucleic acids respectively.