The 15 one-mark questions from the May 2022 12th Standard Chemistry public exam, in paper order, with the correct option marked and a short explanation.
Q1
An element belongs to group-15 and 3rd period of the periodic table. Its electronic configuration would be :
- A. \(1s^2\,2s^2\,2p^6\,3s^2\,3p^2\)
- B. \(1s^2\,2s^2\,2p^4\)
- C. \(1s^2\,2s^2\,2p^6\,3s^2\,3p^3\)Correct
- D. \(1s^2\,2s^2\,2p^3\)
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).
Q2
Bauxite has the composition :
- A. \(Al_2O_3\cdot nH_2O\)Correct
- B. \(Fe_2O_3\cdot 2H_2O\)
- C. \(Al_2O_3\)
- D. None of the above
Explanation. Bauxite, the chief ore of aluminium, is hydrated aluminium oxide, \(Al_2O_3\cdot nH_2O\).
Q3
If 75% of a first order reaction was completed in 60 min, 50% of the same reaction under the same conditions would be completed in :
- A. 35 minutes
- B. 20 minutes
- C. 75 minutes
- D. 30 minutesCorrect
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.
Q4
Which of the following reagent can be used to convert nitrobenzene to aniline ?
- A. ZnHg/NaOH
- B. \(Zn/NH_4Cl\)
- C. Sn/HClCorrect
- D. All of these
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.
Q5
\(HO-CH_2-CH_2-OH\) on heating with periodic acid gives :
- A. MethanalCorrect
- B. Methanoic acid
- C. \(CO_2\)
- D. Glyoxal
Explanation. Periodic acid cleaves the C–C bond of a 1,2-diol; ethylene glycol gives two molecules of methanal: \(HOCH_2CH_2OH + HIO_4 \rightarrow 2HCHO + HIO_3 + H_2O\).
Q6
A complex in which the oxidation number of the metal is zero is :
- A. \(K_4[Fe(CN)_6]\)
- B. \([Fe(CN)_3(NH_3)_3]\)
- C. \([Fe(CO)_5]\)Correct
- D. Both (b) and (c)
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.
Q7
Which of the following can act as Lowry-Bronsted acid as well as base ?
- A. \(HPO_4^{2-}\)Correct
- B. HCl
- C. \(Br^-\)
- D. \(SO_4^{2-}\)
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.
Q8
An aqueous solution of borax is :
- A. basicCorrect
- B. neutral
- C. amphoteric
- D. acidic
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: \(Na_2B_4O_7 + 7H_2O \rightarrow 2NaOH + 4H_3BO_3\).
Q9
Which one of the following is an example for homogeneous catalysis ?
- A. Hydrogenation of oil
- B. Manufacture of ammonia by Haber's process
- C. Hydrolysis of sucrose in presence of dil.HClCorrect
- D. Manufacture of sulphuric acid by Contact process
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.
Q10
The formation of cyanohydrin from acetone is an example of :
- A. electrophilic addition
- B. nucleophilic substitution
- C. nucleophilic additionCorrect
- D. electrophilic substitution
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.
Q11
Which of the following oxidation states is most common among the lanthanoids ?
- A. +5
- B. +4
- C. +3Correct
- D. +2
Explanation. Lanthanoids commonly lose the two 6s electrons and one 5d (or 4f) electron, so +3 is their most common and stable oxidation state.
Q12
Faraday constant is defined as :
- A. Charge required to deposit one mole of substance
- B. Charge carried by 1 electron
- C. Charge carried by \(6.22\times10^{10}\) electrons
- D. Charge carried by one mole of electronsCorrect
Explanation. One Faraday is the charge of one mole of electrons: \(F = N_A \times e = 6.022\times10^{23} \times 1.602\times10^{-19} \approx 96500\ C\ mol^{-1}\).
Q13
Which of the following amino acids are achiral ?
- A. Proline
- B. Alanine
- C. GlycineCorrect
- D. Leucine
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.
Q14
The crystal with a metal deficiency defect is :
- A. ZnO
- B. NaCl
- C. KCl
- D. FeOCorrect
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.
Q15
Fog is colloidal solution of :
- A. liquid in gasCorrect
- B. solid in gas
- C. gas in liquid
- D. gas in gas
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).
