The 15 one-mark questions from the March 2020 12th Standard Chemistry public exam, in paper order, with the correct option marked and a short explanation.
Q1
Match the following:
(1) Fluorine — (i) Identification of coloured metal ions
(2) Borax — (ii) Strong oxidising agent
(3) Aluminium — (iii) Chalcogens present in volcanic ashes
(4) Sulphur — (iv) Most abundant element
- A. (1)-(iii), (2)-(ii), (3)-(iv), (4)-(i)
- B. (1)-(ii), (2)-(i), (3)-(iv), (4)-(iii)Correct
- C. (1)-(iv), (2)-(iii), (3)-(ii), (4)-(i)
- D. (1)-(ii), (2)-(iv), (3)-(i), (4)-(iii)
Explanation. Fluorine is the strongest oxidising agent; the borax bead test identifies coloured metal ions; aluminium is the most abundant metal in the earth's crust; sulphur (a chalcogen) occurs in volcanic ash.
Q2
Wolframite ore is separated from tinstone by the process of:
- A. Electromagnetic separationCorrect
- B. Smelting
- C. Calcination
- D. Roasting
Explanation. Wolframite is magnetic while tinstone (cassiterite) is non-magnetic, so they are separated by electromagnetic separation.
Q3
The transition element which has only +3 oxidation state is:
- A. Ni
- B. Mn
- C. Cr
- D. ScCorrect
Explanation. Scandium (3d¹4s²) loses all three valence electrons to give the stable Sc³⁺ ion, so it shows only the +3 oxidation state.
Q4
The medicinal value of a drug is measured in terms of its:
- A. Deoxyribose
- B. Gold number
- C. Therapeutic indexCorrect
- D. Equilibrium constant
Explanation. Therapeutic index is the ratio of the lethal dose to the therapeutic dose; it measures the medicinal value (safety) of a drug.
Q5
The aqueous solutions of sodium formate, anilinium chloride and potassium cyanide are respectively:
- A. acidic, acidic, acidic
- B. acidic, acidic, basic
- C. basic, acidic, basicCorrect
- D. basic, neutral, basic
Explanation. HCOONa and KCN are salts of a weak acid and a strong base, so they hydrolyse to give basic solutions; anilinium chloride is a salt of a weak base and a strong acid, so its solution is acidic.
Q6
If one strand of the DNA has the sequence 'ATGCTTGA', then the sequence of complementary strand would be:
- A. TACGRAGT
- B. TACGAACTCorrect
- C. TCCGAACT
- D. TACGTACT
Explanation. A pairs with T and G pairs with C, so ATGCTTGA pairs with TACGAACT.
Q7
Which one of the following is most basic?
- A. 2,4-dibromo aniline
- B. 2,4-dichloro aniline
- C. 2,4-dimethyl anilineCorrect
- D. 2,4-dinitro aniline
Explanation. Methyl groups are electron-releasing and increase the electron density on nitrogen, while –Br, –Cl and –NO₂ withdraw electrons; hence 2,4-dimethyl aniline is the most basic.
Q8
How many Faradays of electricity are required for the following reaction to occur: \(MnO_4^- \rightarrow Mn^{2+}\)?
- A. 7 F
- B. 5 FCorrect
- C. 3 F
- D. 1 F
Explanation. Mn changes from +7 in \(MnO_4^-\) to +2, gaining 5 electrons per ion, so 5 F are required per mole.
Q9
Assertion: p-N,N-dimethyl aminobenzaldehyde undergoes benzoin condensation.
Reason: The aldehydic (–CHO) group is meta directing.
- A. Both Assertion and Reason are false.
- B. Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
- C. Both Assertion and Reason are true but Reason is not the correct explanation of Assertion.Correct
- D. Assertion is true but Reason is false.
Explanation. As an aromatic aldehyde without α-hydrogen, p-N,N-dimethylaminobenzaldehyde takes part in (crossed) benzoin condensation, and –CHO is indeed meta directing, but its directing influence in ring substitution does not explain benzoin condensation. The item is loosely framed (the electron-donating –NMe₂ group hinders self-condensation); (c) is the intended answer.
Q10
Laptops have:
- A. Lead storage battery
- B. Fuel cell
- C. Mercury button cell
- D. Lithium-ion batteryCorrect
Explanation. Laptops and mobile phones use light, rechargeable lithium-ion batteries.
Q11
Formula for hyponitrous acid:
- A. HOONO
- B. \(H_2N_2O_2\)Correct
- C. \(HNO_2\)
- D. \(HNO_4\)
Explanation. Hyponitrous acid is HO–N=N–OH, i.e. \(H_2N_2O_2\); \(HNO_2\) is nitrous acid.
Q12
Williamson synthesis of preparing dimethyl ether is a/an:
- A. Electrophilic substitution reaction
- B. \(S_N1\) reaction
- C. \(S_N2\) reactionCorrect
- D. Electrophilic addition reaction
Explanation. The methoxide ion attacks the methyl halide in a single step from the back side, displacing the halide: an \(S_N2\) reaction.
Q13
The vacant space in bcc lattice unit cell is:
- A. 26%
- B. 48%
- C. 23%
- D. 32%Correct
Explanation. The packing efficiency of a bcc unit cell is 68%, so the vacant space is 100 − 68 = 32%.
Q14
Time required for the reactant concentration to reach one half of its initial value is called:
- A. half life periodCorrect
- B. first order
- C. zero order
- D. second order
Explanation. By definition, the time taken for the concentration of a reactant to fall to half its initial value is the half-life period.
Q15
The major product obtained when phenol reacts with Con. \(H_2SO_4\) at 280 K is:
- A. Salicylic acid
- B. Picric acid
- C. o-phenol sulphonic acidCorrect
- D. p-phenol sulphonic acid
Explanation. Sulphonation of phenol at low temperature (about 280 K) gives mainly o-phenol sulphonic acid; at 373 K the para isomer predominates.
