12th Standard Chemistry May 2022: Online Practice Test

Explanation. A group-15 element has 5 valence electrons (\(ns^2np^3\)); in the 3rd period n = 3, so the configuration is \(1s^2 2s^2 2p^6 3s^2 3p^3\) (phosphorus).

Explanation. Bauxite, the chief ore of aluminium, is hydrated aluminium oxide, \(Al_2O_3\cdot nH_2O\).

Explanation. For a first order reaction, 75% completion takes two half-lives (100 → 50 → 25). So \(2t_{1/2} = 60\) min, giving \(t_{1/2} = 30\) min for 50% completion.

Explanation. Reduction in acidic medium (Sn/HCl) converts nitrobenzene to aniline. In neutral medium (\(Zn/NH_4Cl\)) it gives N-phenylhydroxylamine, and in alkaline medium bimolecular products such as azoxy, azo or hydrazo benzene are formed.

Explanation. Periodic acid cleaves the C–C bond of a 1,2-diol; ethylene glycol gives two molecules of methanal: \(HOCH_2CH_2OH + HIO_4 \rightarrow 2HCHO + HIO_3 + H_2O\).

Explanation. In \(K_4[Fe(CN)_6]\), Fe is +2; in \([Fe(CN)_3(NH_3)_3]\), x + 3(−1) + 0 = 0 gives +3; in \([Fe(CO)_5]\), CO is neutral so Fe is 0.

Explanation. \(HPO_4^{2-}\) can donate a proton (forming \(PO_4^{3-}\)) and accept a proton (forming \(H_2PO_4^-\)), so it is amphiprotic. HCl can only donate; \(Br^-\) and \(SO_4^{2-}\) can only accept.

Explanation. Borax is a salt of a strong base (NaOH) and a weak acid (boric acid); it hydrolyses in water to give NaOH, so the solution is basic: \(Na_2B_4O_7 + 7H_2O \rightarrow 2NaOH + 4H_3BO_3\).

Explanation. In the acid hydrolysis of sucrose, the catalyst (dil. HCl) and the reactants are all in the same (aqueous) phase. The other three use solid catalysts (Ni, Fe, \(V_2O_5\)/Pt) with gaseous or liquid reactants, so they are heterogeneous.

Explanation. The nucleophile \(CN^-\) attacks the electrophilic carbonyl carbon of acetone and HCN adds across the C=O bond, so it is a nucleophilic addition reaction.

Explanation. Lanthanoids commonly lose the two 6s electrons and one 5d (or 4f) electron, so +3 is their most common and stable oxidation state.

Explanation. One Faraday is the charge of one mole of electrons: \(F = N_A \times e = 6.022\times10^{23} \times 1.602\times10^{-19} \approx 96500\ C\ mol^{-1}\).

Explanation. Glycine (\(H_2N-CH_2-COOH\)) has two hydrogen atoms on its α-carbon, so it has no chiral carbon; the other amino acids listed are chiral.

Explanation. FeO is found as \(Fe_{0.95}O\): some \(Fe^{2+}\) ions are missing and electrical neutrality is kept by \(Fe^{3+}\) ions, which is a metal deficiency defect. Heated ZnO shows a metal excess defect instead.

Explanation. Fog consists of tiny water droplets (dispersed phase: liquid) spread in air (dispersion medium: gas), so it is a liquid-in-gas colloid (aerosol).